There will be some kind of relationship between the percentage of rakeback PokerStars were to give and the softness of the games on Party. If PokerStars were to begin increasing its rakeback above 0%, more and more good players would go over to Stars, which would in turn increase the softness of Party's games.

I think it would be tough to pick an exact number because your decrease in earn at PokerStars due to a tightening of your average player will not stay constant as the percentage of rakeback offered goes up.