I have seen several posts that indicate the mag of the sky in the viewing
area. How is this determined? Is it subjective?

Yes, it's subjective. Several folks have helped by explaining how to find a
single faint star to determine your naked eye limiting magnitude (NELM).
Another method is to count the number of stars you can see within a specific
area of sky. Here's a link to a site with charts and tables to help you convert
your star counts to a NELM: http://www.seds.org/nineplanets/lm/rjm.html

Regards,

Bill Ferris
"Cosmic Voyage: The Online Resource for Amateur Astronomers"
URL: http://www.cosmic-voyage.net
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