Thanks for the clarification. OP edited for clarity, and a rather embarrassing mathematical error.

demonfork wrote:We already have this, it's called "relative rank" and it's retarded.

Where can I find this? And is it strictly based on percent won vs average opponents, or something else (how is it calculated) ?

Concise description:

Create an extra stat in player's profile, offering a percentage of won games, including adjustment for how many players participate

Specifics/Details:

If a player wins a 6 player game, his percentage will adjust, just like he won 5 different games. If a player loses ...