How is your grade decided?

Champion masteries seem to be an awesome idea, and you have my total support on this idea :D A few questions concerning the grade that you get based on performance on the end of a match.. how do they decide the rank? Does it rank on k/d/a regardless of champ? If I did play support as janna or something and went 0/0/10 would I be given a good grade? even 0/2/10? Or is it something deeper than k/d/a?
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