When measuring your wheel outside-diameter using the above method (noting where the tire stem comes to ground twice), do it while sitting on your bike with your full weight on it, in normal riding position. This will compress your tires slightly and give the exact same result during calibration, as you get when riding the bike.

As seeker333 said, you might have to get someone to help when doing this, holding the bike upright and pushing you while he walks alongside and marks each exact position where the stem "touched" the road.

If you simply walk beside the (empty) bike and mark where the stem hits, you will be off by just a smidgen. Maybe not enough difference to matter. But hey, if you want to do it "right"....

Cateye computers come with a very complete list of tire sizes, and what calibration number to use for each. I had one on a road bike, and put in the exact number they recommended for the 700Cx25 tires on it. I then rode around the bike path on the local lake (Miramar), which has mileposts every 1/4 mile. I know, there's no guarantee that those posts are exactly correct. But I noticed that the first post (1/4 mile) and the 3-1/4 mile post, were EXACTLY three miles apart according to that Cateye computer, right down to the foot.

I figure that either the Cateye computer and the mile posts were off by the exact same amount and in the same direction... or else they were both correct. Choice B seems more likely to me. So I've been using those two mileposts to calibrate every computer I've had, ever since.