When I teach short fields t/o and landings, I make sure to use the performance charts for the aircraft. Then state that you might make it in but you might not make it out. I also teach to add a safety margin onto those by the book numbers as I am usually teaching on a 30 year old aircraft, the numbers dont really reflect reality. So, once you practice the shorts, you can add a little onto the airspeed for windshear, because you should never actaully be using a strip the same length the performance charts said you could get into. The variables change to easy, temp, weight and wind especially.