The repeater compass in the German bombers is just a ... repeater. Not a gyro compass. It reproduces (more or less accurately) the information from the magnetic compass, to a more convenient area (i.e. in front of the pilot's eyes).
The difference to true north consists of the magnetic deviation (10 degrees to add to the reading) plus or minus a compass error induced by magnetic interference in the airplane (electrical equipment, metallic masses, etc.). Plus or minus wind drift of course...
This number varies depending on the heading of the plane. Every plane has a "variation chart" which indicates the correction by heading.
In the Bf 109 it is displayed to the right of the cockpit.
For the other aircraft it has to be done by rotating the aircraft, reading the heading on both compasses and comparing them to true heading.
Below is a summary chart for most bombers in CLoD (BR20 still WiP). Blenny data courtesy Rostic.
Compass corrections v20191226.pdf
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