There's no calibration and all the compass does is to make sure the drone can fly straight.
It plays no part in position accuracy.
RTH position accuracy is entirely down to the (variable) accuracy of consumer GPS.
Much of the time it will be within a few feet but sometimes it can be 10 feet or even a bit more.
You don't have to let RTH land for you though.
You can cancel RTH and resume control to choose a landing spot better than a blind GPS unit can.