before reading the 15% min is an assumed number and may be incorrect.
The way the equation is done is my best guess based on a previous game.
There are 2 checks that happen for a debuff the first is chance to activate that is the % in the description. The second takes your accuracy and the targets resis into account then determines the chance for it to apply. There does appear to be a min chance for it to stick regardless. So if you have 0 accuracy you still have a chance for it to stick same goes for having 0 resis. The last game i played like this it was tested out to be 15% chance to stick or resis. I have no idea if its that number in this game but to illustrate what looks like is going on here are few examples that may make it clearer but may also be wrong since i don't have the testing or the equations for this game so keep that in mind.
so with the above example you have 75% chance of activation and 120 acc. Lets assume your target has 100 resis. What is most likely happening is 120 - 100 which means you would have a 20% chance after the second check. if the target had 110 resis that would be 120 - 110 = 10% but if the game had a minimum set to 15% you would then default to 15% instead of 10%. if the target had 50 would then have 120 - 50 = 70% chance for the second check to make the debuff stick.
this is how it seems to play out. you cast the spell/attack the first check happens with a 75% chance if falls in the 25% fail then thats it if its successfull it does the math 120 - 100 = 20% chance to stick since the all the resis was canceled out they have 15% chance. If it falls in the 80% chance to not stick its resisted if it falls in the 20% chance to stick it then rolls again with the 15% chance to still be resisted.
assuming its 15% min then 0 acc - 0 resis woudl be 15% on both sides 15 and 15 would be the same they dont cancel out. 20 and 0 would still be 15 since the defaut is used so 20 - 0 would really be 20 - 15 = 5 and your still defaulting to 15% . at 30 - 0 you get 15 but its wasted since your still at 15%
If this is how it truly works then it would make sense to have over 30 acc before it makes a difference assuming the target has 0 resis. We also dont know if there is a hard or soft cap on acc and resis. For those who don't know what is a soft cap is when you get diminished returns on points into that sat so instead of it going up by 1 for every point it goes up by .5. A hard cap just means it wont go any higher so anything above that is completely wasted.