This is equal to the brightness of the faintest star that can be seen on the sky. I usually estimate this by looking for the faintest star near the zenith, that I can see with direct vision with my bare eyes. As conditions are changing frequently, I try to pick a star that I can continuously see, not only here and then. Then I check on a star map the exact brightness of that star, which gives the NELM of the moment.

   NELM changes by time, so I usually re-check it at every sketch.

   A NELM of anything under 4m I consider poor, above 5.5m is good and above 6.5m is excellent. Of course this is my personal rating, it maybe different for you.