What a load of crap. I hope no early PPL or student is reading this thread and thinking any of it matters. Heres a top tip - if you ever find yourself thinking that flying a couple of knots slower will mean that the certain crash you are about to have will now become a close, but somehow heroic, shave you honestly need to get out of the plane and go and have a beer to make sure you dont get back in the plane for a bit. Its a load of flight testing nonsense. Let me explain how ridiculously stupid it all it. Nobody ever bothers to wind the gradient. In a rubber band powered single, the wind component will very significantly improve or degrade your climb gradient and it will do it by orders of magnitude more than messing about with Vx and Vy. Forget all about it because it just doesnt matter.