I feel like it's 80% weighted by CS. And sivir is commonly a high CS champion.
So if you had 270 cs @ 27 and like 0/0/1 that is probably S- rank.
I'm not kidding. Every CS per min you stray from 10 per min is a harder S rank. It's still role and champion dependent, but sivir ADC would be like the highest. CS even helps in the support role imo. And if you're a support with 50 cs it helps your grade.
This is assuming you are using your trinket and control wards, but you should anyways. They say that helps but tbh I think it hardly matters compared to the CS really.
I consider 8 per minute the minimum decent CS for any lane champion. 9 per minute being good for S on a decent game.