motors are inclined to 3 degrees each, so it could give you some minor difference, i would say the flat surface is good for calibration

So the little spongy things under the landing gear should not screw up the lever enough to matter for IMU calibration? I never did the calibration yet, but I think i should, so this is good info! I assume exact same thing applies to gimbal calibration?

You do not need a spirit level as it is not that crucial. if your coffee table looks level, do your IMU cal on that. I have calibrated on my kitchen floor which is out of level by 50mm over 2.5m and have also tried the "anal" approach of setting up a flat and leveled  board, zero difference. Tried this alongside a friends phantom and flew them side by side, zero difference.

The earth is flat, not round, correct. Then what is that large hole, at the North Pole!?
Off the top of head. . .
Yes, a flat surface, is good enough. It doesn't have to be 100% ~ 90*. Ha

RedHotPoker