How much do enemy helmets reduce damage by?

Order triarii are by far the tankiest of the basic (excluding flame rats and interfectors) order enemies, specifically the ones with helmets cause god damn. Ive made my own gun rebalance mod, and the plasmois rifle does 80 damage now.

these shits still take 6 shots to the head to kill. thats for the ones with helmets. The ones without die in 3 hits, which makes sense since they have 175 hp in the head and 80*3=240. However, helmeted grunts have the same hp on their head, 175, but it takes 6 shots, which is 480 damage. So how exactly do helmets work for enemies? Reduce damage by percentage (based off damage and hp im guessing around 60% [40% of 80 is 32, 32*6=192)?

those are thick ass helmets. i also noticed that the plasmois rifle doesnt knock helmets off bru.

but also i shot one of them 3 times in the torso, in which they have 200 hp. He shouldve died but he didnt, so idk.