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 a few sets with you and repair as needed. Most monuments have repair benches for this reason.
I had another look and I do agree the durability loss is a bit high and came to the conclusion that the damage NPCs deal are much higher than that of a player, so the mathification from damage to durability loss is not going to match what you'd receive in pvp anyways. I'm working on it now to balance things a bit more