B&G Deckman - Manual (Page 121)

Chapter 7: Instruments
7.16
Figure 7.10
The port and starboard boat speed factors shown in the calibration
display (Figure 7.10) are used to provide additional calibration to that
given on the instruments. In particular if you feel that you are having a
problem in getting the boat speed to read the same on each tack—
possibly because the paddle wheel is off centre—then each tack can be
corrected separately. In the example the boat speed would be reading too
high on port tack and so it is reduced by 2%; it is too low on starboard
and is being increased by 2%. The actual value of the correction
Deckman applies depends on the angle of heel (if it is specified): the
maximum correction is used at the maximum angle of heel and it
reduces smoothly so that when the boat is upright no correction is
applied.
The apparent wind angle calibration is used to correct for errors in mast
alignment. This should be corrected on the instrument system itself—the
facility is only here for the sake of completeness. A positive correction
makes angles larger on starboard and smaller on port.