It is based off your last 30 games. It doesn't restart every 30 games. Say you have played 30 games and you played your 31st, then the first game of that 30 would drop. I am not sure how it really works because I have never quit a game and I started out at 90%. Not sure why that is but I have played some games with people that were at 84% that said they haven't quit either. So not really sure how they are calculating these last 30 games but wish they would look into it and find out why the percentages are not adding up to what others are saying.
I also think that the % affects the one that did not quit. If you are playing joeblow and he quits, I think it affects your % as well. It should not affect the other person, but I think for some strange reason that it is affecting both. Reason I say this it because I personally never quit a game but have had many quit on me. Looking at my % at 90% and me not quitting tells me that my % was affected by the other player quitting.