Potency based on health spells question

Do the potency based on health spell gems use max health or current health?

Current Health.

Ok thanks, one more question:
Why can acid rain spell gems not get the stat potency enchant? Should it not increase the debuff?

That was an oversight - it should be able to have that property. I manually flag all the properties that each spell can have, so I must have overlooked that one. I’ll fix it in the next patch!

Ah ok good to know