NavList:

Paul Hirose you wrote: "I've never seen that before and have not figured out
how the precomp is arranged to make that work."

Paul

This is my take.  It which says more or less the same as Frank, but I’ve written it, so I’ll post.  You can do this because time, as we know it, is calibrated with respect to the motion of the stars and as near as damit to the mean motion of the Sun & Planets i.e. 15 degrees per hour or 15 minutes of arc per minute of time (see the ‘Increments and Corrections’ page in the nautical almanac).  Not so the Moon, which is always a bit behind the game i.e. 14.3 minutes of arc per minute of time.   So, if you take your shots four minutes early or late you just move your assumed positions one degree east or west depending upon whether you’re early or late.  All you must do is remember which way to move it. Hence the well know cricket expression, especially in the UK, LBW, Leg Before Wicket, or Late Back West.  What it does is ensure the LHA you go into AP3270/HO249 with remains the same, so you can use your original pre-comp. If you’re running late at M0.84/480kts it means things aren’t going too well, so the last thing you need to be doing is getting your AA and AP3270 out.  If you’re using three, five, or even seven shot, two-star sandwich fixing, it’s easier, because you only have two assumed positions to move, but sandwich fixing does mean your fix is somewhat out of date by the time it’s plotted on your chart.  For three-star fixing, if you keep your shots four minutes apart, you can still do it, but you need to move three assumed positions. 

How far would a Moon line be out for a four-minute move of assumed position?
14.3’ x 4= 57.2’.  Therefore, a Moon line would be out by about 2.8’ E or W.  On a one in 2 million plotting chart, that’s about the thickness of a very blunt 2B pencil or a very sharp bar of chocolate.  DaveP

Maybe I’ll fill a pre-comp form in for you, but what might have taken me two minutes 40 years ago will probably take about two hours today, so no promises.