As mentioned above, electrical or steel structural inteference will play havoc with your compass settings. My advice to you would be that once you find a 'quiet' location, do the calibration once and leave it alone. The risk is doing a crap calibration and watching your drone either disappear in the distance or plough into the ground. I've only done one calibration in over twelve months despite flying hundreds of klms from home, and even when prompted (often near my steel framed house), I'll check my immediate surroundings.