Tony Flanders wrote:

Certainly, playing around with the program the Schaefer
published in S+T some while back indicates that NELM varies
much more slowly than sky brightness.


Yes.

Nils-Olof Carlin has written a web page about Schaefer´s paper at
http://w1.411.telia.com/~u41105032/visual/Schaefer.htm
If you scroll down a little, you will find a table giving the
limiting magnitude for different sky backgrounds, both according to
Knoll/Schaefer and Blackwell/Clark. Behold.

Cheers -- Harald