I think there is too much voodoo going on regarding compass calibration, and compass sensitivity.
While the calibration should be done at a place far from metal, there is no problem (as far as I know) flying in an area with metal, or taking off a concrete slab with rebar.
Also, there is no need to re-calibrate all the time: Just when changing location more than 100 miles.
If anyone thinks I am wrong, let's discuss this here and make sure we get to the bottom of this. I believe that people are misleading by suggesting that drifts are caused by flying near metal. You would have to be really close to the metal, for it to impact the compass. Too close for it to happen while flying, unless you are a daredevil...