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Gary, you asked:
"I don't follow the discussions of lunar distance very carefully. What
is the expected accuracy of the derived longitude in practice?"

If you're doing lunars for their traditional purpose, namely to find GMT and 
thence longitude (when compared against local time) then the accuracy in 
longitude is approximately 3' of longitude (12 seconds of GMT) for ever 0.1' 
error in the observed distance. Modern observers can get that level of 
accuracy with well-adjusted good quality metal sextant using a 7x sextant 
--IF they average four or more observations. Historical observations vary by 
decade and other factors. Lunars were just about perfected right about the 
time when they were no longer needed.

You can also shoot lunars at KNOWN GMT to get a position fix (this was not how 
they were used historically). This is a fix that does not require any type of 
horizon, which is its principal advantage. In this case, you get both 
latitude and longitude from a pair of lunars and the accuracy in each is 
about 6 n.m. per 0.1' error in the observed distance.

-FER




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