On an Android device you can obtain:

(a) From the magnetometer: The device orientation (i.e orientation relative to N).
(b) From the GPS : Bearing from current location to a specified other location.

Both of the values are returned as the number of degrees E. of North.
So they are in the range of +0- +180 or -1 to -179 (-180 might be possible - haven't checked)

I want to put an arrow on the screen pointing from the current location to the specified location (in the range of 0-360)
What is the most efficient algorithm ?

Actually the magnetometer reading if for Magnetic N. and the bearing is True North but I can adjust for that later (same for the device window orientation)

I can see a few implementations on the net but I've a feeling that they could be simplified..........

Suggestions ?