Raymarine ST6001 - Zanshin

Chapter 6: Commissioning the Autopilot 67
Adjusting the heading alignment
If you experience difficulties with compass alignment, you can check
the compass alignment after completing the deviation correction
procedure (swinging the compass). After completing the initial
compass calibration, you can make further adjustments to the
alignment without swinging the compass again.
Although the compass calibration removes most of the alignment
error, small errors (of the order of a few degrees) will probably
remain. These will vary depending on the heading.
Ideally, you should check the heading reading against a number of
known headings, plot a deviation curve, and determine the heading
alignment value that will give the lowest average alignment error.
You can then enter this value on the Heading Alignment screen, as
described above.
If the average heading error is more than 5°, you should perform the
compass deviation correction procedure again, circling more slowly
and in more favorable conditions.
6 Commissioning the Autopilot