Champion grades transparency

Attached is my most recent game, in this game I felt like I played really well won denied hella cs from there adc. Transitioned into midgame with some great roams on there mid yas. In the end game I had the second least deaths on my team the most assists and the most damage all playing as support zyra and I was awarded a A- for my contribution where everyone else who was taking advantage of my zoning and snares got S's or S-'s. These odd grades never used to bother me until I was trying to get to rank 7 as zyra, I will have games where I just stomp there bot lane and transition into a complete carry for my team and only get a A or an A+ if Rito is feeling generous. This leaves me wondering what is the nitty griddy of your guys grading scheme, please shed some light on what you guys deem required by a mage support in order to achieve the coveted S rank!

Please and thank you rito!