Here's a couple of write-up's on the subject.
https://tonyflanders.wordpress.com/a...ght-pollution/
http://uncle-rods.blogspot.com.au/20...y-how-big.html
I would agree with Uncle's statement about M13 and light pollution 6"v12"... Except that with my skies and 5.11" to 8" aperture test on Omega Centauri the 8" showed it brighter and resolved more of the stars.