Well without getting too techincal (and id rather not), it would be just your own 'judgement'. I played medivac through entire games to my loss, and getting a high level medivac isnt that easy when you have good opponents. So getting high % heals may not be likely. In my case, healing average was much higher (obviously) on high levels but by that time, everyones HP was so much higher that it was just a waste of effort, and i would only heal for the pretty effects

even if im full hp.
If you want a really good medivan / medic tank / healer/supporter take a look at team fortress 2's medic. Im quite leet with him and its way intense and hectic like playing a combat class (apart from engi). The fun would come from saving allies and constant support for them which is done in the most minor way through the ulti.
Anyway, heals are too small early due to not enought creeps bunching together, so i suggest making it targetable on allied creeps. I still wouldnt bother much with it though, its just doesnt work very well no matter what values its given.
I would ask for someone to make a table of calculations of heal % vs number of creep but i dont expect the skill to stay the same for long.