All headings measured on the map are true. So you need to add 10 degrees to obtain the magnetic heading to fly both on line or off line. Your dg can be aligned on any refrence true or magnetic. However since the only refrence you have in the cockpit is the magnetic compass the convention is to align the dg to a magnetic refrence.
So measure in true convert to magnetic, set dg refrence magnetic heading and fly magnetic.
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