Apple has a calibration process that doesn't require a flat surface. The first time you go to the "level" app (it's a right swipe from compass) - you are asked

Apple has a calibration process that doesn't require a flat surface. The first time you go to the "level" app (it's a right swipe from compass) - you are asked to spin a little virtual ball in a circle (kind of like a roulette wheel) - after three or four spins it's calibrated and (at least on my iPhone 5) works as a dead level.