Ignoring defence

A little off topic, but how would Chaos PB even work. Since, for example, I thought 40% PDR meant that you deal 60% of your full damage, would 180% PDR mean you would need ignorePDR/Boss %/Total damage that makes you deal over 180% of your full damage in order to do any damage to Chaos PB?

If I'm not mistaken, defense ignore isn't additive to their defenses. 100% def ignore would mean you ignore 100% of that 180%, less confusingly you would ignore 100% of their defenses... unless it changed, but I don't recall any change.

If it's 180% defense, and you have 40% defense ignore, you would ignore 72% because 40% of 180 = 72.