Mag calibration, health color, and correctness use both the direction of the mag field (for vehicle direction) and the strength of the mag field (to try to verify there is no disturbance in earth's field).
If you start with the mag being calibrated correctly and if you don't get the magnet too close to the vehicle, this might be useful, but the only reasonable use for this would be to get the mags green so you can arm where you would not normally be able to arm, like indoors, and only for non-flight, indoor testing, or later carrying it outdoors for flight. In essence, you would be getting the mag field strength correct (so health is green, so you can arm), but not the direction.
To start in a location where there is a disturbance, and place a magnet to fix both direction and strength at once is basically impossible without some very fancy equipment. Even with this, since the magnet you use is only detectable in a very short range, the area that is corrected by it will be very very small, so moving the vehicle or magnet even a short distance will undo the correction. Just not reasonable except in a lab.
Then there are AC magnetic fields because of AC wiring in the house. The mag field changes constantly. To see an example, hold a fan (like desktop fan) close to an old style (CRT) TV set or computer monitor. Where I used to work, there was an area around a power panel (including high current air conditioning) where you couldn't use a CRT.