I have always taken off on top of my tonneau cover on my truck bed. It's aluminum but the truck frame is steal. I have never had a compass calibration warning. That's been with the spark, mini, mini 2 and 3, air, air 2s, and now the air 3s ( which I may be selling shortly )
Only time I have gotten a warning about the compass was when I was launching from the ground at glass butte in eastern oregon, probably due to the mineral and iron ore content around
Starting and launching near ferrous metal is a potential compass bias problem, not a calibration problem. No warnings will appear to indicate a bias problem.
To avoid a bias problem and possible uncontrollable flight, just confirm that the compass reflects the actual drone orientation before lifting off.