Simrad AP28 Operator's Manual page 119

Zero point setting
Unless you need a rudder angle display when leaving the
dock, just steer the boat manually on a straight course
and press the AUTO key. The zero point is then set
automatically.
If you prefer to use the rudder angle display when leaving
the dock, proceed as follows:
Turn the wheel from lock to lock (H.O. to H.O.) and count
the exact number of turns. Then start from one lock
position and turn the half number of turns.
Press AUTO and then STBY. The zero point is now set and
STBY
AUTO
the display will show:
After turn-on the rudder angle
display will alternate between 10
degrees port and starboard to
indicate that the (rudder) zero
point need be set.
Return to the operation section.
There is no further need for zero
point settings until next time you
turn the autopilot on.
Optional equipment | 119