Just slightly changing the subject a bit here. I did my PPL a few years ago and can't remember that I was actually taught short field techniques! Am planning a trip down to the South West during the summer and assuming you have all your calcs correct for t/o, is there a standard allowance you make for all the variables ie. heaviness of ground, length of grass etc. In other words, would you only depart if, when you've done all your calcs you still have a spare, say 100m?