Do you mean two-point? If you mean two-point while jumping, you want to wait for the horse to close your hip angle. I've found that it helps to close your eyes before the jump, if it's a small one, so you can't anticipate it at all. I'm sure other people will have suggestions as well :). If you mean half-seat, there really is no correct time for this (early or late, I mean). Usually in equitation, most people will ride a half-seat canter around the course. In other disciplines, too, of course :). I THINK you mean two-point, but if you do mean half-seat other people can give you more suggestions :).