the math has always been the same in all games like this..
stacking % additive damage is always worse.
if i want to double my damage i need 100% additive, if i want to double it again i need 300% additive.
but i can also double it by giving the enemy -100% lightning resist
that's a 200% total vs a 400% total..
very simple
It all depends what the enemies resistances are. Most monsters you encounter with -res on gear and conviction applied will be in the case 3 area. Where super uniques and bosses will be more case 1/2 area. We have been pking these bosses for years besides heralds with no charms and had no issues. I aim for clearing speed so case 3 is the best overall option for fastest clearing.
Case 1: High resist enemies
Enemy = 75% resist
You deal 25% damage
Add -25% res → enemy now 50%
You deal 50% damage
You just DOUBLED your damage
Case 2: Lower resist enemies (smaller gains)
Enemy = 0% resist
You deal 100% damage
Add -25% res → enemy now -25%
You deal 125% damage
Only a 25% increase
Case 3: Deep negative resist (tiny gains)
Enemy = -50% resist
You deal 150% damage
Add -25% → now -75%
You deal 175% damage
Only a ~16% increase