NavList:

Geoffrey you said:

how far does the assumed position have to be from the true position before the error due this approximation starts to become equivalent to the 'average' radius of the Ellipse of Confidence?

Like I said earlier in a different thread, it doesn’t do to be too inquisitive.  The more stones you upturn the more variables seem to pop up and worry you.  E.g. what about incorrect azimuth lines to plot your PLs along?  That kind of depends upon how far away the two star’s sub stellar points are. If you go to the same LHA for two successive latitudes in AP3270, HO249 Vol 1 (i.e. an assumed position error of the order of 60nm N-S), you’ll see that the azimuth of low altitude stars (low 20s) rarely change, medium altitude stars (30s-40s) change by about one degree, and higher altitude stars change by one or two degrees.  It’s not always obvious, because the azimuths in these documents are rounded to the nearest degree.  Then start drawing little diagrams of the effect on your fix.  This appears to also depend upon the orientation of your stars the the line joining your assumed to you real true position.  After a bit your head starts to hurt, well mine does.  DaveP