I don't think we have an exact system but when we first started we are allowed to make up scores for written tests and practical tests and so for people who gave themselves high grades like 82 and 93 they are in the higher A-Z ranking so most are in C ranking and up except for my character who scored a 55 and a 92 and so I would think it proper to put her in a L rated class, she's strong, just not book smart. But it's all up to the GM and if he wants I could make a proper scale for grades and rankings.