Calibrating the digital compass is not necessary nor is it recommended. The suggestion to hover and allow your P2V to acquire GPS satellites is OK but even better is setting your P2V in an open area, turning it on, and leaving it there without moving it until you get green lights and you see that you have sufficient satellites for a home lock. Remember proper start-up procedure (this assumes you have already performed the first-time set up digital compass calibration properly):
1- Turn on radio/transmitter
2- Turn on wifi range extender
3- Set P2V in open area, turn on and allow it to initiate start up and acquire sufficient satellites for a 'home lock'
One other safeguard is to make sure you have updated firmware. DJI addressed the fly away issue with a firmware update. CEO Colin Guinn explains it
http://www.youtube.com/watch?v=2bxjL7wFyb8 It's actually a pretty interesting and clever fix.