Correction, once per projectile. Bind it to Devastation if you don’t believe me.
Your dmg calculation for PB dmg is incorect, sheet calculates it per blade and every blade caries 30% weapon dmg. And why would you have 26/12/0 PB line? Also if Bat would be 50% PB dmg everyone would take it 
Also even if your calculation is right you would have to bind twin fangs to PB itself so you would have to drop acids spray.
Edit: Ofc you can go with devo setup you proposed its solid and I had exact same one, but changed it in order to cap stun resistance.
Edit 2: Not exact same one I had lizard instead of hound.
I did the maths based entirely on Grim Calc values. Level 26 PB gives you 80% wpn damage, but the transmuter gives you -50% wpn damage -75% total damage. In the end you have 30% wpn damage per blade, but that is further reduced by 75% total damage.
6 blades *30 wpn dmg *0.25 total damage transmuter=45% wpn damage if all blades hit.
I see why you’d want Acid on PB with Witch Hunter, Trickster has it easier due to Wendigoo and Swarm. But you can easily put some points in either Sigil or Doombolt and you’ll have 100% uptime on bosses.
Dmg of PB per blade for this build is 2.6-2.9k after all reduction calculated, you can see that on picture I posted with PB toll tip dmg breakdown.
Even for trickster having acid spray on on PB is much better than having twin fangs.
Can you do me a favor and make a screenshot while hovering over the Bat constellation in the Devotion tree? I want to see it’s damage with all the bonuses your character has.
Sry I didnt had time earlier to explain a bit more so I wrote just one sentence.
Here are toll tops of TF and AC:
Dont let that 68% confuses you BF bound to PB has 20% proc chance, I was experimenting a bit before respecing out.
So as you can see TF per fang has 2667-3807 dmg and goes of 2 times per sec.
Comparing sheet dps of single phantasmal blade with basic dmg of the blade you can calculate that average number of casts per sec is 2.74 so in 0.5 sec interval single PB does 2.740.5(2616~2966)=3584-4063.
Following this you can calculate that single fang to single P blade ratio is 0.74-0.94. When you multiply it with total number of fangs and devide by total number of blades you would get 0.25-0.31 or in percents 25%-31% increase in total PB dmg when you bind TF to it.
In mean time Acid spray reduces total resistance by 30 and compared to current c speed (2.74 cast per sec) every P blade (6 of them) will proc it averagely in every 2.43 sec. calculating its dmg to 0.5 secs interval it comes to 668,93 dmg per blade per 0.5 sec interval (actually is more cuz poison is dot dmg type but for sake of argument lets say that this is correct number).
Since there are 6 blades total dmg of acid spray in 0.5 secs interval is 4013 to every target that is hitted by phantasmal blades wich is higher than single twin fang. Only when bot fangs hit same target TF gives higher paper dmg, I say paper because also worth mentioning is that acid spray is quality vise better dmg than twin fangs cuz you have higher value of acid poison rr than pierce for example and pretty similar like for vitality.
I didnt even calculated 30 flat dmg reduction 
Your math confused me a bit. I’ll try to simplify it a bit - what will happen over 4 seconds.
Ideal non-realistic calculation:
Twin Fangs will ideally proc 8 times. Assuming both fangs hit that is 823230=51.700 dmg
Acid Spray will ideally proc 5 times. (51600=8000) acid + (4825=3300) (poison dmg over 4 sec)=11.300 dmg
===============
More precise calculation. 2.47 casts per second means 1 cast every 0.4 seconds. The time between casts is enough so that the CD of either ability will be refreshed and no proc opportunity will be wasted.
Chance to trigget Bat with 6 blades is around 74%
51.7000.74=38258 dmg -> around 1/3 of this damage is pierce
Chance to trigger Acid spray with 6 blades is around 62%.
80000.62+3300=8260 dmg
If both fangs hit the target Bat is much better then Acid Spray DPS wise and you get bonus sustain out of it. Now, clearly, you need to get that RR from somewhere. This means either Black star of deceit (can you cap PB without badge?) or putting Acid Spray on another delivery tool. The RR from acid spray lasts for 5 seconds, so a Doombolt + Acid spray will give you 100% (or close to) RR uptime on bosses.
Again I dont have time to explain but you got it wrong. Every blade has independent chance of procing acid spray and independent CD for that devotion ability cuz it procs on hit, for record same goes for twin fangs.
P.S. Again your calculation is wrong.
Edit: also much more than 1/3 is some other dmg (ie pierce, chaos,…) cuz of your weapon dmg component.
Edit 2: Dom bolt to proc acid spray is a terrible idea, sigil also.
Edit 3: Also black star is terrible idea, procs on crit, has 10 flat less and have smaller range.
Finally: I will stop this now, cuz dont have time for writing more. If you find twin fangs better bind it to PB. And if you are not to lazy check on Fabius kill times with both setups (you dont have enough energy for dummy).
Again I dont have time to explain but you got it wrong. Every blade has independent chance of procing acid spray and independent CD for that devotion ability cuz it procs on hit, for record same goes for twin fangs.
P.S. Again your calculation is wrong.
I am not sure if that is correct, but even if we assume it is that benefits Bat even more due to higher base damage, higher base proc chance and lower cooldown. The resistance reduction on Acid Spray doesn’t stack and neither does the poison.
Still, I’d love to hear your arguments on this when you have enough time to write them 
Edit:
Edit: also much more of 1/3 is some other dmg (ie pierce, chaos,…) cuz of your weapon dmg component.
As per your screenshot -> Vitality damage is average 2071 damage, pierce is 900 and the weapon mix is 266.
Hey, I craft a PB-build around Acid as Main type, but i miss a lot of items to test it. Did you think it worth to farm them, or is this build shitty?^^
(Not sure with the Devotions, too)
Is it worth to stack a lot flat acid damage if i use PB? (like i do)
Phantasmal Blades Acid-Chaos-Vitality Witch-Hunter
Basebuild: http://grimcalc.com/build/1007-CueVBc
With all + skills (see below): https://grimcalc.com/build/1007-ebwgNh
Damage Types:
Main: Acid
Secondary: Chaos, Vitality
Head: https://gracefuldusk.appspot.com/items/9086-Cowl-of-the-Venomblade
Shoulder: https://gracefuldusk.appspot.com/items/8972-Moosilauke’s-Pauldrons
Chest: https://gracefuldusk.appspot.com/items/9067-Stealth-Jacket-of-the-Venomblade
Hands: https://gracefuldusk.appspot.com/items/7925-Viperfang-Grips
Legs: https://gracefuldusk.appspot.com/items/8230-Nidalla’s-Legwraps (Better something green)
Feet: https://gracefuldusk.appspot.com/items/8013-Venomspine-Greaves
Main-Hand: https://gracefuldusk.appspot.com/items/8179-Quillthrower-of-Dreeg
Amulet: https://gracefuldusk.appspot.com/items/8348-Pestilence-of-Dreeg (Affix: Plague of Shattered Souls)
Medal: https://gracefuldusk.appspot.com/items/8362-Badge-of-Mastery (+5 to Phantasmal Blades)
Ring 1: https://gracefuldusk.appspot.com/items/8871-Viloth’s-Ring
Ring 2: https://gracefuldusk.appspot.com/items/8871-Viloth’s-Ring
Waist: https://gracefuldusk.appspot.com/items/8211-Spellbreaker-Waistguard
Relic: https://gracefuldusk.appspot.com/items/2945-Malediction
These items give me:
+2 to All Skills in Nightblade
+2 to All Skills in Occultist
+4 to Amarasta’s Blade Burst
+8 to Phantasmal Blades
+2 to Vulnerability
+2 to Blood of Dreeg
+2 to Lethal Assault
+3 to Phantasmal Armor
thx fo your statements!
well devouring swarm or Cof is so good for pb but lets be honest devouring swarm will totally reduce your clear speed a lot, cause you will prolly wanna cast it most packs. cof on the otherhand not the best curse for pb again, lacks chaos RR…
i got your point and yep i tested twin wangs… its like overkill tbh. damage output is awesome but decent okish geared pb char is kinda unkillable already. twin fangs life leech with tons of dmg is minor thing you would want on your pb char…
on the other hand acid spray procs nearly every single shot of pb, and its %100 reliable with zero effort 30 flat RR… chaos + vitality + pierce + cold + bleed heck even aether if you have all gonna be boosted via RR,
but twin wang will boost just proc dmg + constellation bonuses are super minor…
and acid spray pathing is better with oa + %4 hp.
i am not man of math in arpgs, i simply go and test where i used to farm and acid spray outperforms twin fang so hard vs hard enemiesbosses nemesis
meanwhile twin fang boost generall clear speedlike pb needs more clear speed?.
its your call at the end, i find acid spray kinda mandatory for smooth playstyle but your char -> your taste & decision
ps: even akeron scorpion flat DA reduction on enemies outperforms twinfang so far imo.
Azmodael, Acid Spray will stack with other sources of RR.
RR bound to Blades is much better, i do the same on my Blades of Nether caster (Cool name for Blood Orb PB Caster, huh? :D).
And this isn’t just because each blade has an individual proc chance, i’ve generally noticed RR bound to the main attack adds to the effectiveness on any build.
It is correct, and again test it and you will see. Check your kill/clear times and we will than talk again
Sry not much more,was looking lower dmg but still 37%
This also, specially cuz manticore is not just single target rr.
JoV, devs have seen your build and decided Demonslayer’s Garb Set is too OP, so need to nerf it
Well they decided to nerf almost all builds that I posted so noting special about this one also
But in light of all changes I cant resist the feeling that this particular build will be maybe even buffed
Yeah, more pierce damage and vitality convert are very nice, but Demonslayer set is weakest legendary set in game, crits is the only one that saved it, also -OA from Eye of Beronat.
No.
10/chars
i disagree with that , its not the best for sure but set design is all around really awesome to me… and its really powerfull imo…
i kindly disagree mate
Poor resistances, pistol for Nightblade, no + to mastery.
What set is weaker then??
Torzan, Invokers, Barelsmith…