[S]upport Rating

I was wondering if anyone knew if the end of match ratings had changed recently or if it was mentioned in an recent News and I missed it? Since ARAMs now give you a personal score at the end of the match, I was also wondering if changes had been made to Normal matches?

The reason I ask is because I'm trying to Level Sona to 7. Months ago (prior to L6 & L7 when L5 was the highest Champion Mastery) I used to get S ratings when I played Supports, almost every game.

As of late, since Level 6/7 Masteries were introduced, I've noticed that I frequently receive A+ or S- ratings. Recently I received an S+ for a 10.3.33 match, and someone told me I could only get an S or higher if I had 30 assists. I was hoping that isn't true, considering some matches are over entirely too quickly for me to even get 30 assists. (I wouldn't normally have so many kills as a Support, but it was a 3v5 for about 10 minutes, and I was having to play super aggressive until those players returned)

I place wards everywhere, and usually ALWAYS have the most wards placed at the end of the match. I've had long matches where I've placed around 50-60 wards. But even then, I'll get an A rating. I've had matches where we dominated our lane early, like within 10 minutes, and still got an A rating. In those matches, I can recall having around 20 assists - so it would make sense that you can only get an S rating with 30 assists.

But that doesn't make sense either, to be graded off a flat number - since not all match lengths are equal.

Anyone have any insight? I just need 1 more token, and I'm not sure what I should be focusing on to get it. It's been like 3 S- ratings in a row, and I'm getting pretty bummed. :(