How about this variant: regeneration is equal to Max(Archangel basic HP (MA:P(MON_ARCHANGEL)/?(value:y), 20% of BM:H).

Advantages:

1) The artifact mostly fully heals any normal 1..6 level creature and even 7 level creature without experience or life bonuses. It's even not bad for commanders and henchmen.
2) If script multiples all creature health by some value, like 2, it will be accounted. For instance, basic archangel health will become 500.
3) The artifact is not overpowered. It does not fully restore azure dragons health, commander health or henchman health.
4) It still scales for super HP creatures like commanders and henchman. If commander or henchman has 3000 HP, he will regenerate 3000 * 0.2 = 600 HP, which is strong enough.