Not sure about Preemptive, but the Degenerative -Max HP debuff is 2% per application, up to 4 stacks even from different casters (no more than -8%). Additionally, I understand it will never debuff more than 150 HP total, but that's a second-hand testimony.
Basically, the HP debuff sucks balls right now, so if you're going for Degenerative, focus on the 75% Toxic DoT side of things.