Thanks for running my numbers Gary, it's satisfying to the the confirmation. I hadn't looked closely at precession/nutation corrections yet. The examples in the introduction to Pub 249 don't show it's use, nor do my older references(Weems, the Navy Air Navigation series, and H.O. 216). Do you think the '40s navigators (the guys I'm mostly trying to emulate here) would have been applying this? They were using H.O. 214 and 218 then, were these published annually with data current to the years equinox?
And two more Polhemus questions:
1) In using the 45° overlay I moved N41 up three latitude lines to get some plotting space from the edge of the disk. Am I correct observing that this should not create any error when plotting a single fix?
2) Reading Lat - I assumed that one dosen't follow the Base grid from a point on the overlay to the Lat scale, but that one follows the curve of the Lat graticule to the Lat scale. Is this how you do it?
HO 214 is similar to volumes 2 and 3 of HO 249 so they are permanent and are not updated. This works because you use the actual, current, GHA (SHA plus GHA Aries for stars) and declination of the body while HO 249 volume 1 uses the coordinates of the selected stars for the "Epoch" year, spaced every five years with the P&N correction tables to all its use for plus and minus five years from the "Epoch" year.
HO 218 is a mix of HO 249 volume 1 for stars and of HO 249 volumes 2 and 3 for solar system bodies. The selected star section used the SHA and Declination of the stars in 1941 and includes a correction method for the altitude of each star (NOT A SINGLE CORRECTION FOR THE FIX) usable to the year 2000. You can also use HO 218 similarily to HO 249 volumes 2 and three for stars and this is the same method as used when working stars with HO 214. So if you want to emulate the use of HO 214 then just use HO 249 volumes 2 and 3 for all the sights, including stars.
To the precision available with the Polhemus, there is no difference in the fix when using a latitude parallel up or down a few degrees. And you are right about following the parallels to the central meridian to determine latitude of the fix.