Andre Faria: I think the ratings should be based on a curve, so that the difference between 1st and 2nd is greater than between 2nd and 3rd etc.

This would obviously be modified by the opps ratings.

For example a simple hyperbolic formula 1/position could be multiplied by the opps average rating and used to calculate your new rating.

new = (old * n + (2/pos * oppRating * numPlayers))/n+1

where n = a modifier that increases with games played to a point.

I wish I had the data to try it.