In my opinion which can or cannot be right.

If you look at poison fog wave (Water main - 1st mystery)
it says "poisoning causes ninjutsu damage to the maximum life percentage"

I have pretty low 48k power (lvl 83)
when I was fighting with enemy "A" (55k power, lvl 87) poison dealt to him 4.8k dmg at 4 stacks - target Iruka approx. 11k HP
meanwhile in fight with enemy "B" (63k, lvl 91) poison dealt 5.4k dmg at 4 stacks - target Iruka approx. 12k HP

both had pretty much same lightning blitz team
"A": Lightning, Iruka, Asuma, Danzo
"B": Lightning, Iruka, Asuma, Darui

therefore my conclusion is:
- poison deals 10% HP per stack (up to 40% in total)
- poison stacks have hard cap of 1999 dmg per stack (up to total 7996 dmg)

Resistance or Ninjutsu will not play a role in this. But as I said, this is my opinion.