Once your skill gets over 20 you get a mastery so 21 would be 1M, you get a mastery every extra 20 skill levels so 56 would be 16M2. Masteries bump up your success, once you're at a crit it bumps down the opponent's result. Masteries cancel out before rolling. (15M3 vs 3M becomes 15M2 vs 3, really going to suck for that 3M guy).