How is Champion Mastery Score Determined?

Can anyone explain how riot determines what score you get for your champion played at the end of a match? Playing as finishing with a score of 15/6/8 (which I know isnt fantastic) taking 4 towers and 139 cs (also not crazy) only nets me an A ranking. I'm not saying it was an S+ worthy match but at least A+ or S- , maybe I'm missing something here. It would be great to know the parameters that they use to score with.