Hi Jan,
Another value you can check out using the Telemetry view in QGC (with a recently released firmware) is the "ACC Magnitude". This should be as close as possible to 9.806 when the board is in any orientation. If you hold the board steady on each edge for bit, you should see this value settle down to 1g fairly quickly. In the Values Chart view, you can also see a running average of this value, which can be helpful. If you see differing values for different sides, that is an indication of poor calibration. At least this is how Bill explained it to me once
"If you find that this number varies a lot from one orientation to another, you need to re-calibrate your sensors."
Similarly, the "MAG Magnitude" is useful for checking the magnetic calibration. Bill says, "It is should be calibrated such that the Earth's magnetic field registers as exactly 2.0."
One effect I noticed from imperfect ACC calibration was that when flying in automatic modes, my quad would slowly loose altitude as it traveled horizontally. PID changes had no positive effect, and Bill suggested checking the calibration via the above method. Sure enough, it was off... I was getting readings in the ~9.8 to 10.02 range, depending on orientation. My other board which didn't have the altitude problem gets readings in the ~9.93 to 9.78 range, which Bill said also wasn't great.
None of my calibration attempts have been especially careful... though I do let the calcs run until the MAE stops dropping. Now that the weather is getting decent, it's time to dust off the dancing shoes again and try to get some better flights with the quad.
Cheers,
-Max