NavList:

One neat trick is it turn the boat slowly but within a few minutes. Even if you don't know the azimuth you have the Fourier coefficients for compass correction except  the constant term. Much easier than transits and pelorus. Billĺ

Lets assume variation = 0 and deviation = 0 on all headings, and the suns azimuth is 135. Therefore, the shadow of the top of the compass disc will lie across 135 on the disc.  What I think you’re saying Bill is that if coefficient C was five degrees, the deviation on N would be  5E (it might be 5W, but I’m not going to argue at ¼ past bedtime), so the compass disk would be pulled 5 degrees clockwise and the shadow would lie over 130.  On south, deviation would be 5W, so the compass disc would be pulled 5degrees anticlockwise, and the shadow would lie over 140.  Subtract 130 from 140 and divide by two to get coefficient C.  I might have got the E and W back to front, but it’s 35 years since I last taught compasses.
I can remember when Hartlepool Marina was the Coal Dock.  The Reserve Fleet was mothballed on one side, and they were still exporting best British coal from the other.  The railway sidings from the collieries were kept high, and the coal trucks had a trap door in the bottom.  A chap pulled a lever and the coal slid down a chute straight into the ship’s hold.  One of the other docks was the Timber Dock.  Ships used to come in from Sweden and Norway piled high with a deck cargo of pit props for the collieries.  There was also a Timber Pool where they floated the props until they were ready to use them.  Every Christmas they left the branches and needles on one tree and sent us a full size Christmas tree.  Those were the days.  DaveP