The amount of durability you lose is based on the amount of damage you receive, offset by the protection the armor provides. The % of loss is distributed based on where you're hit. So if you're shot in the chest by a scientist, that will absorb the durability loss.
It's designed exactly how it is in vanilla for PvP with the exact same rates. That's not to say it's perfectly balanced, but at the same time this is quite literally the only thing that causes durability loss to your armor. Bring