Compasses are 180 degrees out

[Buzpilot]

I also noticed in Bf109 E series, the gyro moves opposite direction, so if you turn 90 degrees to the right, it moves wrong way, and when it settles it moves back 180 degree
So it actually show correct heading, but its animation is wrong.

[IvanK]

You need to first determine the current magnetic heading from the Main compass. Once you have this set this on the directional Gyro.

Gyro precession is modelled so every 15mins or so you need to repeat the process. You also need to do this after hard manoeuvering.

To further complicate things acceleration and turning errors on the magnetic compass are modelled so you need to take this into account as well.

Magnetic variation is NOT modelled in COD