Any of the rollout tests or formulas will work plenty well.

BUT -- the lazy man's approach is to google "bicycle tire circumference"

You'll get many hits; the first is a link to Sheldon Brown's discussion on the topic, which includes a table showing the data.

I'll save you the trouble:
http://sheldonbrown.com/cyclecomputer-calibration.html

(HINT: ignore the "group" descriptions at the top of the table and instead look at the bottom of the table to interpret the columns. The circumference in mm is the far right column)

Or you can follow the other links and get to similar (but often slightly different) results. But they are all close enough.

You can tweak the setting later if you choose, but don't fret a couple of percent error...

-TH