IMU calibration & compass swing are two totally different events!! IMU calibration needs to be done in a quiet place away from any vibration on a known plumb/level surface & done so after the machine has been off for a good period of time so the circuits are cold. I mean don't even walk around the machine while the IMU calibration is running! The compass swing should be done at every flight location but not before a second or third battery at the same location.
Your compass sensors should be somewhere around:1400-1600 hope for a mean of 1500. This reading will trend in a given direction when exposed to a field. What I mean is watch your readings, normally (1496,1504,1492,1510,1501,1506,1498,so on...) they appear to be somewhat random but nonetheless hover right around a mark say 1500 +\-100. if the compass sensors are introduced or removed from a magnetic field the readings will still seem somewhat random but they will trend in a direction higher or lower. (1496,1505,1499,1508,1500,1515,1502,1513,1522,1514, so on...) the readings will continue to do this until they reach a neutral balance & begin to hover around a number set.
The IMU sensors on the Inspire are the most sensitive I've seen, simply shuting the door in the same room as you may be running the IMU calibration can ruin the calibration & make you have to recalibrate. Outside, performing an IMU is very easy because of the sun exposure & expansion of material making pops, creaks, also the springs winds will ruin the calibration. So I place my Inspire inside on the kitchen counter when I have the house to myself & the dogs are asleep, washing machine, dish washer, etc. aren't running & it's deathly quiet in the house for 5 minutes. I can't say what exactly it does while calibrating but my guess is, it shuts down each axis, allows it to cool for a minute then powers the specific axis IMU, samples the readings & zero's out the sample. It does this for X,Y, & then Z axis, you can watch you readings 0 out in that order, but who knows for sure.