Well that's what I do too...when it's late at night running into headwinds and bad weather, all the FBOs are closing down, I don't look at my FMS or pull out the flight manual for long range fuel, I start calculating TAS X 1.32 divided by PI, the Earth's ecliptic squared, subtract the cups of coffee left in the pot behind me, divided by a calculated ounces of bagged cubed, multiplied by the amount of bug splats on my windshield.
