Is it necessary to calibrate before every fly even if the home point is the same?
AC

I usually recalibrate when I change to a new location different from my previous flight.

+1 for only at a new location...

I done mine when I got it new and again when I upgraded firmware to 128 .. iv been 70 miles away from where I done it the second time and didn't calibrate and it didn't make a blind bit of difference .... I don't calibrate ever but that's just me ....

No, it's not necessary to calibrate every time.
The manual is confusing, contradictory and badly written in the compass section.
Extensive flying testing by many experienced owners has shown that you don't need to calibrate the compass every time.
In fact you can happily and safely go months without recalibrating at all and to unnecessarily recalibrate a perfectly good Phantom is introducing innecessary risk.
Read this to get an understanding of what compass calibration is all about and you'll fly safer:  http://www.phantompilots.com/thr ... plete-primer.32829/

I  only calibrate after an update or when I change locations.

KEvans Posted at 2016-4-15 11:20
I  only calibrate after an update or when I change locations.

Even then it is probably not necessary. The only reason for recalibrating the compass is to take into account magnetic variations in position, and quite often there is no variation for hundreds of miles around you.

I have only calibrated my compass once, about 6 months ago when I got the P3, and I have travelled all over my state for hundreds of miles and it made no difference at all.