If you want to get really technical the correct thing to do would be to use a spectrum analyzer to see if there are any other devices broadcasting in the 5ghz range and what channel and width they are using.  99% of the people don't need or want to go to that effort so I'd try 80 and if things seem unstable then go to 40.  I run 40 in my house but I also have three different devices broadcasting 5ghz.