What are the Champ Mastery Grading Parameters? -roughly-

While this isn't a topic i'm particularly passionate about, i am curious as to what factors affect my grading at the end of game and i was wondering if anyone has hashed out exactly what is expected of S+ ranking. While i understand that riot doesn't want people to game the system to artificially inflate their scores, a grading system doesn't do much if they don't tell you what you did good and why you got the score you did.

If grading is meant to encourage us to improve our gameplay, shouldn't we be privy to what it is we need to work on? Rather than getting a vague letter grade on about how well we did, wouldn't it be better to say click the Grade we got and see a potential breakdown of our strong and weak areas? Learning new champions can be painful sometimes, especially when you're learning a new role or trying something new. Likewise, while we know that the system is suppose to judge us based around where we go with champs and the roles we use them for, wouldn't it be nice for the grade system to be able to tell us who we're being judged against? On the off-chance that somehow our stats are being skewed by a mis-labeling?

Anywhom, this isn't a thread to bitch about Champ Mastery, but more of a question on what it looks for in determining how well someone did. I don't take the grades too seriously, and find them pretty cute sometimes, but i'd just kinda like to know more about them lol.