Getting S or S+ on Kindred seems impossible.

Has anyone else had a similar issue with them? I feel like I've been doing VERY well recently on Kindred (specifically my last game, albeit it was a troll game.)

Going 8-2-7, and getting 16 marks in 24 minutes seems like an S to me. But no, its an S-. While I got my lanes ahead and chose to let them have kills over taking them myself; I dont understand why that should get an S-.

What do I have to do to get an S on this freakin champion? She's massively feast or famine, and when you're stupidly far ahead and end the game at 24 minutes with 16 marks and excellent farm, you still get an S-.

Any input on what it takes?