But if it flew fine then leave it that way because it is still in the stable range. The reason for balancing it empty is that as your tank empties the CG will move aft and you don't want it to end up tail heavy at the end of your flight. If the tank is right on the CG you don't have to worry about this. If the tank is aft of the CG you would balance the plane with a full tank. Apparently 4" back is OK for this plane.