Ok seriously what's wrong with your ranking system riot?
I just went 10/4/6 on malzahar as support with smite (there was a lobby mix up) and got an S+
Where as i've gone mid malz with an end score of 18/3/XX and only managed S...
Seriously, if you're going to attached rewards like the hextech chests to them, it'd be nice to have a rough idea of how they are given. Even schools require a rubric prior to the assignment. Even if it's just arbitrary ratios between different objectives and kills.