I said:

Oh how I long for a cheap, widely available device to give an
objective measure of sky brightness! As things stand, we are
like the people building the tower of Babel, all talking at
cross-purposes to each other.

Bill Ferris responded:

I'd say Clark, Schaefer, Carlin, Bartels and other have done an
excellent job of speaking in the same language. And they share
similar motivations and goals: to help us better understand how
we see under low-light conditions and what our limits of vision
under those conditions are. And they've had some significant
success.

Yes, I would agree. And as for your own chart of NELM - sky brightness,
I am sure that your 22.0 mag per square arcsecond figure is quite
reliable, since it is totally objective except for some possible
quibbles about spectral distribution. And I am sure that *some*
people can see mag 8.0 stars under such circumstances. I doubt
that I could see much fainter than mag 7.0, however, so that isn't
much help for me. For me, NELM seems to stop being a useful
measuring device for any skies much darker than 20.5 mag per
square arcsecond; after that, my NELM bottoms out. I get much
more useful results by seeing what diffuse objects are visible,
which does *not* bottom out. But it also isn't quantitative.

Moreover, although I am happy to accept your 8.0 - 22.0
correspondence for an important set of experienced observers,
I suggest that this does *not* extrapolate to 7.0 - 21.0,
6.0 - 20.0, etc. Instead, I suggest a curve more like this:

8.0 - 22.0
7.0 - 20.5
6.0 - 19.0
...

The only way to tell for sure is to take one highly conscientious
observer and get NELM estimates under various conditions, with
a good photometric device at hand to get simultaneous measurements
of sky brightness. Actually, this should probably be tried for
multiple observers; there is no reason that the shape of the
curve should be the same for all.

Thanks to the Moon, it should actually be quite easy to get
measurements under various conditions of sky brightness.
Starting at a dark site, you don't have to travel anywhere;
just wait for different Moon phases.

But even if you can derive such a curve, it isn't necessarily
helpful for the average moderately experienced amateur, whose
NELM estimate may be quite different from, say, O'Meara's.
That is why, in response to the question "what should I expect
to see under my skies", the best I can usually say is that
you can see what you can see, and probably more if you try
harder.

The closest I have come to an objective measure of light
pollution is to observe the skies at various Moon phases.
If the sky is very little worse at full Moon than at new,
then you have very bad light pollution. If the sky is
blatantly worse when a 5-day-old Moon is up than at new
Moon, then you have pretty decent skies. But that is
an exceedingly crude measure.

- Tony Flanders