This is the scenario:
You travel 20kms from the last "good" compass calibration point.
At the new site, you don't notice that there is more interference (rebar, more high voltage, cell towers etc).
You calibrate thinking it's required.
You take off and she spirals and circles all over the joint.
What do I suggest:
Don't calibrate unless you have travelled a significant distance. eg >50kms, another state, another country or if you have sub woofers in your car.
Get your satellites and take off and see if she goes TBE (toilet bowl effect). If she does, then calibrate. If no symptoms of TBE - have fun.
Need to remember also that the DJI app will warn you if a compass calibration is needed.
Another option would be to bring a laptop/windows pad and check the compass MOD (1300-1600) is within range.
A few things I have noticed, If I take off on man made structures/concrete, the compass calibration required warning always appears. I pick her up and take off using my case/back pack/wooden table and the warning goes. What I think is that the compass is way too close to the ground where it is heavily affected by the geomagnetic forces and therefore the compass goes berserk. I will do some field trials, taking off at 5/6ft (hand take-off using an assistant - boat launch technique). I will then see if she TBE's at close locations.
My phantom has been suffering from TBE from day1. This is due to the magnetic declination. Australia east coast is 10 degrees off true north.
Remember, this is only a suggestion.