Hm. Skills let you make a formula based on max and current hp, but only for the exact moment the skill is used.
States don't have a formula window... so theres no way to control its regen over time amount without a plugin. Perhaps Yanflys Buff and State Core plugin... but at first glance it would be complicated.
like. it has a notetag
<Custom Action Start Effect>
code
code
</Custom Action Start Effect>
which you put on the state, and then where the words "code" are you would put your formulas. So every time the actor has a turn start, the formula would run.