When I calibrated my computer the first time, I just tossed my hiking GPS onto the bike and ride to work. Then I see how much they disagreed by, then added that. For instance, if the GPS said 10.5 miles, and the bike said 10.1 miles, and the computer was set to 2150mm, I'd change the setting to 2150 * (10.5/10.1) = 2235

Since my route is entirely consistent, I know that when I get to work every day I should have really close to 10.47 miles on the trip odometer. When I put on new tires, I just check that. So far I haven't noticed any significant difference between different 700c tires, including both Nokian and Schwalbe studs.