Back when I cared whether it was really accurate or not, I did my first calibration with a rollout and some chalk, then I rode about 20 miles with a hiking GPS velcro'd to the bars and assumed the GPS was going to be pretty accurate. At 20 miles it was about 0.2 miles short, so I adjusted the value up by 1%.

These days I don't worry about it all that much, I just use the same value of 2155 on my old bike regardless of what wheel I put on it. I did do a rollout on the new road bike, and that came to 2095 (the old bike generally has 32 to 35s on it, the new one, 23s)