there is actually a fraternity on my campus that has a rule (i'm not exactly sure of the "mathematical" equation), but they have a certain time limit until which a brother can date another brother's ex... it goes somehow by the amount of months the couple was together and i believe that if the relationship lasted over a year they don't "allow" any dating of ex's at all. we are a pretty small campus so i am surprised that this method leaves them with any girls left to date!! :> )