If I am looking to utilize ADCTH as a form of sustain, it must be linked to something that does Weapon Damage – right?
So a skill like Bone Harvest, which superficially seems to be some odd summoning spell, would actually allow me to leech – right?
And so even if a specific skill does not have an ADCTH component, as long as my character screen lists some (say, from one of my items, such as a ring or a weapon), then I can use that to leech from any skill which uses weapon damage (even such as Bone Harvest) for sustain – right?
thanks much
Right. On all counts.
Just 1 note about the last point. If a skill does 100% or more damage, you’ll do the ADCTH percentage listed on the weapon used to attack (you might have a difference between main and offhand ADCTH), but if it goes below 100%, the ADCTH percentage will be that of your listed ADCTH * (skill’s weapon damage%).
Oh wow I thought it was direct correlation- 20% weapon dmg is good for 20% lifetap. Guess I thought it worked the same as resist rebuffs. The way you describe makes more sense 
The best lifetap you can get is AOE attacks that have weapon dmg on them, those will heal you up in no time.
It does work the same as resist debuffs. If the skill does 10% weapon damage, and you have a 20% RR on your weapon, you’ll debuff the mobs by 10% * 20% = 2%.
It’s especially bad for ADCTH, because you are also applying that reduced ADCTH on reduced damage.