The player I picked up a couple days ago is at 79% and I have only sold 4 players in the past 14 weeks.
Well I'm at 80% and have sold 2 players in the past 14 weeks. The % goes down the more players you have sold.
The max is 97% and I believe you get that if a player has been in your squad for 14 weeks, regardless of how many players you sell in between.
Run of the Mill Canadian Manager