This is extremely useful if you are soloing with no-cost Carby with some way of recovering MP (Spirit Taker). Although the cost is high, it recovers both your and Carby's HP without hate, which is useful if you don't have a way of running away to recast.
Too bad Healing Ruby I cannot target Carby itself.
I'm not sure about the formula, but it heals much more than Healing Ruby I. It's more MP effective with a greater heal than both Spring Water and Whispering Wind(Although no status cures ^^;).