Dark Knight changes needed

[TouchandFeel]

grit reduce damage incoming in 20% and dealt -20%, with darkside (aument damage dealt 15%) the total result is 20% damage reduction and -5% damage dealt (i don't know the exact formula but is close to -5%).

It's multiplicative, not additive so you actually have to multiply the 80% (-20%/from grit) damage being done with Grit on by 115% (15% boost from Darkside) to end up at 92%, not 95%.
(1 - 0.2)x1.15 = 0.92

This works with all stacking buffs/debuffs that affect the same thing (% damage up, % damage down, etc.) in the game.

Also, Galgarion is both correct and incorrect in his statement. The Darkside buff does not reduce the 20% damage loss from Grit, that 20% is always that in the equation while calculating damage in Grit.
However, Darkside changes the equation itself by introducing in another factor to it so while the 20% is still there, now so is the 15% increase from Darkside, which in turn does alter the result of the equation to be a higher number which someone could describe as "lessening the damage decrease caused by Grit" if you focus on the end result.

I somewhat feel that it has become a bit of an argument of semantics, perception and wording.