Glad that you mentioned badges. Expert and Master definitions can be difficult to agree upon. If you need a system that the website can identify, you are on the right track.

For this purpose I believe that Master is the tough one to define. Rookie being something like less than 10 games played. Novice up to 25, or 50. From their you could simply be identified as Player until you reach the high point of Expert. 100 games, or 500 games, or some other high number.

Master should require some special considerations. First, you must be an "Expert". The website would then look at the total of your games (if it has records) to determine win/loss ratio, modified for the number of opponents. It would also look at kill/death ratio. If the system uses an ELO type system, you could simply call the top 5%, Masters. Otherwise, it would probably be easier to just say that any Expert with a win/loss that is 20% above average, and kill/death 20% above average, is a Master. To find an appropriate amount of Masters, I would suggest setting the programming up to do the task, and then plugging in win/loss and kill/death percentages of different amounts until you get an appropriate amount of Masters. Round those percentages up or down to a nice number like 33%, 50%, 10%, etc., and you are done.