Suggestions for the player/hextech ratings system (S,A,B,etc)

I enjoy the hextech crafting system but I would really appreciate Riot giving the players more information regarding what they need to do in order to earn S- or higher ratings. I'm and ADC main, trying to earn my second S for Mastery 7 on Caitlyn. I went 18/0/13, 265 farm 19.7k gold, 17 wards placed, 46.7k damage dealt to champions, this was a 43 minute game. I was given an A+ for this game.

Let me repeat that for those who didn't fully take it in. 18 kills, 0 deaths as an ADC in 2016. Riot please tell the players more directly what we are supposed to do to improve to an S or higher. When I played Zilean a while back I went like 4-7-15 and earned an S. This is so frustrating.