Gaining a greater health from Potions?

For items, the value of healing is based on the “max out” flag with the “affects HP” or “affects MP” flags.

If the “max out” flag is set, the value will be a fraction of max HP or max MP defined by the item power. The formula is X/16, where X is between 1 and 16. The value of 16 is 16/16 or 100%, the value of 8 is 8/16 or 50%, the value of 4 is 4/16 or 25%, etc.

If “max out” flag isn't set, it will use the value defined by the item power. Unfortunately, it is only one byte and can't store values beyond 255. If you want fixed values beyond 255 which aren't related to max HP or max MP, you will need to change the code, assembly style.