GPS Heading - Compass Heading

Question

I have a dial with an arrow on it - I want this arrow to point to a GPS coordinate and adjusting for compass heading at the same time.

I'm getting the compass to rotate just fine, and I can also point the arrow in the correct GPS-heading, but I'm having trouble compensating for the compass heading when rotating the GPS-heading arrow.

Could anyone give me a quick example of this? Been struggling for days!

ATB. Chris