NavList:

You said you were using the Aeronautical Almanac that gives the angle to minutes of arc.  May I suggest you use the USNO Air Almanac ( Free download ) that will give you the same accuracy as the British Nautical Almanac or the USNO Nautical Almanac. The quick interpolation table on the cover gives you arc to minutes but on page A 164 the table  gives you the arc to one tenth of a minute. ( same as Nautical Almanac)

Thanks. I have printed page A 164.

When I started reducing sights I used the refraction table in the aeronautical almanac. Then one day I noticed something strange.  Hs= 33°  01'  -> R = 2'  but Hs = 29° 59'   ->  R = 1'.  Clearly the real world does not work like that. I now use the altitude correction table in the 1973 NA which gives the correction to 0.1'. I am of course assuming that the correction has not changed in the past 43 years (-;

Index error is something else I have become more careful about. Initially I uses IE=0 because it was "close enough". Yesterday I measued IE by the touching limb method and have concluded that it is  2' on the arc. With an AH that is a 1' error in Ho. I did not need to apply an IE correction for the equal altitudes but will of course use it when I calculate the intercepts.