I don't see any numbers from 2013. I just went by what I saw from a couple of 2011/2012 articles.
Just saw this which matches your numbers -
http://www.bizjournals.com/dallas/blog/2014/01/dallas-cowboys-local-tv-ratings-fall.html
from a previous search for 2013 -
http://cowboysblog.***BANNED-URL***...-down-about-10-percent-from-last-season.html/
For 2012
http://www.***BANNED-URL***/sports/...s-biggest-hometown-viewership-drop-in-nfl.ece
I'm no math expert, but the 750k number seems to fit as an average.