Difference between Prima's rakeback and everyone else?

[jokerthief]

From what I've researched prima calculates mgr by how much you contribute to the pot, so if you contribute $20 into a $100 pot you get credited with 20% of the rake for that hand. If you folded preflop you get nothing.

This seems to me that prima's rakeback %'s must be highly inflated compared to other sites that calculate mgr by dividing rake by players in a hand. Does anyone know how to compare the two methods or how big of a difference there is?