This tool should make angle calibration so much easier and more accurate

  1. Attach the drone on the square platform
  2. The platform should have a spirit leveler so, you know its water levelled
  3. Just spin the wheels for the right direction

What do you think?

Another idea would a large wooden box that you hold the UAV against each side to get it level.

A table top adjacent to a wall has worked well for me on all but the largest craft and then the floor and an adjacent wall works fine.