Just for grins, the last time I drove from Eureka to Orange County, 700 miles apart, I did not bother to re-calibrate the compass.
I did this as a test.
OC is +11 deg 41 min (+11.6883)
Eureka is +14 deg 17 min (+14.28333)
That is only a 2.60 degree difference but combined with the distance many would feel the need to recalibrate the compass.
I observed no difference in the flight performance, positioning accuracy, or any other issues between the 2 areas.
This is with an H+ with Build 822/829
This site allows you get your declination (variation) accurately:
Click on the map to find the magnetic declination at your location
www.magnetic-declination.com
This site allows easy conversion:
Degrees (°), minutes ('), seconds ('') to decimal degrees angle converter and how to convert.
www.rapidtables.com