It works based on durability percentage. So you’re correct in that the number doesn’t matter, the ratio of it matters. I’ve easily seen damage numbers drop every time I died on a gear that lost durability with death.
It was a lot more noticeable before the patch that reduced death penalty.
So to be clear if you have 25/100 durability you will get the same bonus as you would if you had 32/125. It’s the ratio that determines your bonus not max durability. With Indes/Unrep combo you will always get the maximum bonus.