OK, time to come clean. I don't do a lot of math in the cockpit either. This kind of exercise is useful as a general guide -- no, if you're over time it is not a good idea to fly off 14000' at VNE. And it's useful to adapt to unusual circumstances. But in practice flying in good air is more important than anything else, and I spend most of my energy looking out the window to figure out where the good air is and the gliders aren't.

John Cochrane