This needs to be calculated as if leaver lost every single round and then scale down to how many matches were done.
Explanation:
Let's say that if I as a healer lost 3 rounds with a player, he leaves, then calculate it as if I had 3-3 and then scale it down (I didn't do 6 matches) to 3/6 (= half) rating gain.
If it's done in a dumb way (you played 3 rounds so let's see your winrate) then people will lose rating just because they were unfortunate enough to be in party with leaver (usually leaver is the worst player, that's why he leaves).
I don't want to lose rating just because a player left and I didn't have chance to be matched enough against him.
This needs to be simply counted as if leaver lost every single round (maybe except the ones he won before he left - so rather as if he lost every following round that weren't played because he left).
I've had so many 5-0s that would've been 6-0. Do you think those two last unlucky players deserve to lose rating just because they couldn't win vs leaver, because he left?
I do not think so. I think leaver knows he would'Ve lost the last match, and he wants to grieve them. Don't make system to grief people.