Hi Joe. The shortest and most direct loop to Watertown is 40.9 miles if you don't mind riding Cainsville Pike up and Statesville Pike back down. They will have more traffic, and I tried to use back roads as much as possible on the main route (and avoid roads that are in bad shape).
Here is a link to the 40.9 mile loop:
http://www.mapmyride.com/route/us/tn/lascassas/589124532939284067
I'll bring a cue sheet for this route Saturday.
If others who are more familiar with these roads have a better 40 mile route suggestion, let me know.