Your formula is extremely pessimistic--and probably works from right in the city among the bright lights.
I favor the Schaefer-derived calculator which works much better for me, observing at a dark site (sky brightness magnitude 21.4 or better):
http://www.go.ednet.ns.ca/~larry/astro/maglimit.html
This calculator has worked for me with several different scopes.
I personally have reached magnitude 15.1 to 15.3 on an average night with my 8" SCT, and have reached (though I haven't really pushed it) 16.4 with my 12.5".
The Schaefer-derived calculator suggests I should reach about magnitude 17.5 with a 20", and I have heard of a few observers going quite a bit deeper with the same aperture, so 17.5 would be quite conservative.
My 5" has reached better than magnitude 14 several times.
I've see conservative formulae that work better for city viewing. Here is one based on Clark's work that is conservative for a dark site (column 1) and conservative for city (column 2)
http://www.twcac.org/Tutorials/limit...tude_table.htm