Question about champion mastery.

After Champion mastery came out I loved the addition of it, just because I'm a numbers junkie. Sometimes though I'll get a score and just shake my head. I've had games where I find myself being like how is that an A? Thinking I did terrible. Others where I'd get an S+ and ask myself how is this one an S+ when this one wasn't? I just had a game like that. Added a pictures for reference, edited names of course. As Blitz I had a score of 0/0/23, which came out to roughly a 60% kill participation. However despite a perfect score, feeding my adc to the point where he was an unstoppable monster, and doing everything I could possibly do to feel like I had a flawless game all around, I got an S- grade; Not that I care so much what the grade is, I just want to know what more it would take to hit that higher grade.

I went back and looked at a match with an s+ rank as Thresh. Score 2/1/32, just short of 80% kill participation. There is a minimal difference in length of the games, and though it should be irrelevant on support my cs is close between the two.

To me, the Blitz game would seem the game that would receive the S+ rating if either was to get it. So finally after taking the long route to ask, is the mastery score based completely on kill participation?

If this has been answered officially and you have a link I'd love to read it. I'm a support main and friends and I have tried to speculate often on what the system takes into consideration. We also think there may be differences depending on the role the champion was intended for, clearly I don't know so hook me up with an answer if you got it!