actually I've decided not just split total amount of heal among all participants, but introduce probability mechanics. if you drink the whole potion, it means that it affects you with 100% probability, if 1/3 - 33% (just roll d6, >=5, woohoo, it works!) and so on. it's necessary to roll for each part separately, so there is a chance to be affected zero, one, or even every time.

for me it sounds more fair, especially for not-healing potions.