Everything I have read says to recalibrate the compass before starting a flight. In the DJI Vision app on my smartphone, I see a little compass on the bottom left with an arrow that points in the direction of the nose of the Phantom. If this compass is lining up with the Phantom, is calibration still necessary? If so, why? Also, prior to taking off, if the distance parameter in the APP is at 0 (or close to 0), is that sufficient confirmation that it has properly recorded it's take-off point as "home"?