Champion Mastery and Hextech Crafting.

With the new hextech system I've been pretty hyped to get 'S' rankings to earn those sweet sweet chests, however I'm struggling to find how you actually get the 'S'. Is there a certain criteria as I have had some good games and not been awarded an S. For example, I just had a game where I played 16/1/11 as Heimerdinger with most damage, most gold (+fully stacked mejai's) and pretty decent farm of 120 for a 30 minute game, this only scored me an A-. (Whilst my premade as singed earned an S with 6/4/14 and 60 farm.) Is there something I'm missing or is this just not enough? Any help would be nice