How to determine the drop chance for an MI w/ specific affixes

alright so i don’t know if there is a difference/don’t remember if someone pointed it out to me prior, but
Since max challenge layer is supposed to give us the best rng out of world farming, i wanted to have kaisan spawn in ToH assuming that’s/Skeleton Key dungeon is the highest available challenge layer we have in regular content/outside SR.
Unfortunately he spawned in the outer parts/before the skeleton key door, but then i remembered the mutator already changes +1 right after (while unsure if that’s actually what hints at extra challenge layer)
So i pulled him behind the gates, and he dropped a double rare neck

No clue if potentially related or not, just regular nice rng, or if the neck is already generated on spawn and not drop (like wysiwyg MIs), or coincidental vs unrelated/related, either way was kinda funny to have happen :sweat_smile:

I’ve come back to his after considering @mi023’s hypothesis in greater detail. I now believe he’s correct. I’ve also updated some of the numbers in the examples for the current patch.