Well the stats are not recorded/available to quite allow my definition.
But basically if you play only in 8 player games then on average you would win an eighth of your game played. Or in 2 player games you win half on average.

so that's average players
winning half that again would be expert in my book

so if you won a quarter of 8 player games
or 3/4 of two player games

or for master try
winning 7 out of 8 two player games (I doubt there are any)

or winning half your 8 player games

or 3 out of 4 of your 4 player games

then you are master in my book, not just an expert