Grade System

Hello, I am curious about how players get their grades after a game.

In the champion mastery FAQ the following is written:

"It’s based on the champion and role that you just played in a game. So, for example, we look at Annie mid differently to Annie support. We take a series of performance metrics and compare how you do to all other players in your region in your champion / position combination. It is percentile driven - so earning a high score means you performed in the top x% of players on that champ in that position. The grade itself governs the point gains."

There are many thoughts what factors influence the grades we get (for example kda, wards/trinkets, etc), but I don't know exactly which these factors are. Can someone link me an OFFICIAL SOURCE in which I can find them? I also read that Riot doesn't want to let us know what the exact factors are and how they influence the grades we get, because it might happen that pleyers would concentrate on improving in those factors and consequently forget about that their goal is to win the match instead of getting an S rank at the end. Is this true? (OFFICIAL SOURCE or Riot worker's confirmation / denial is what I need, not some random answers.)

Thank you!