All of the healing potions heal a slightly variable fraction of your max HP, with some reasonable minimums and maximums, in order to keep healing and extra healing relevant beyond the early game. Proposed quantities are:

• Healing: 8/6/4 HP minimum, 10-20% of max HP, 100 HP maximum
• Extra healing: 16/12/8 minimum, 20-30% of max HP, 160 HP maximum
• Full healing: 100% of max HP, 400 HP maximum