Gaining a greater health from Potions?

You can customize the final value with ff3usme. You only need to change the item power and select the super ball effect. The final value will be the item power * 40. If the item power is 25, the final value will be 25 * 40 = 1000. If the item power is 15, the final value is 15 * 40 = 600, etc.

You can change the multiplier as Madsiur said. I don't know your hex editor but probably the issue may be a wrong address conversion. I use the SNES address convention and you editor may use the absolute byte offset of the file. You can try a tool (like Lunar Address) to do the conversion. Or you can search for the patch bytes with the hex editor search.