I think I know how best to solve the issues with having stronger potions.
We use the percentage per max hp idea, but this means we will have to make all hp potions use the same percentage system.
Like say...
One potion heals 1% of max hp, another potion could heal 1.25% and another will heal say 1.75% and my potion will heal 2% with maybe a level cap, plus cost of each potion will have to be adjusted as well.
Then blitz why don't we just add in a the belt feature, add in an actual belt slot that different belts have different stats. Lower lvl belts could hold 4 pots and top out at 4x4 (16) pot belts on highest lvl belts. Similer to diablo 2. Because let's face it, this game is ironically similer to a handheld mixture of diablo 2 and runescape classic.
I'm new at this whole designing thing, and I've been awake for 36hrs, First I get some good rest, then I give more input on this with a fresh rested mind...