Building a stack does not guarantee getting you to the money in a rebuy. Getting to the bubble is a four hour ordeal. By that time the blinds are 1500/3000 plus antes. What determines how you do in rebuys is how you play for the next 3 hours after the rebuy period. If I have 5000K chips at then end of the first hour, I can make the money by playing good and finish ahead of the bad players who accumulate chips to make up for how bad they will play after the rebuy period. I loosen up a little during the rebuy, but not too much. I maximize my return by doubling against loose players in the first hour and not having to pay more than the original buyin plus a rebuy and add on. Of course sometimes I lose and need to rebuy, but I don't just push on every hand. I don't need a formula for this.