I marked 100 feet to the inch with a contactor's tape measure, then made some faint paint marks on the road in front of my house. It's straight, flat and fairly level, which you need for good calibration.

I put white tape on my front rim for visibility. I start at first mark with rim tape aligned to paint mark, ride slowly to second mark, and stop dead center on end mark. While rolling I count revolutions. At the end mark I determine fractional circumference by counting spokes from tape mark, ie, if I'm 8 spokes past the tape mark to the 100 ft paint mark, then I know the last revolution was only 8/32 = 0.25 revolution. So I completed X plus 0.25 revolutions.

Then conversions. Convert x.y revolutions in 1,200 inches to inchs/rev, which is circumference in inches. Convert to mm or cm as required. Enter conversion factor into computer.

Make sure tires are inflated to pressure you intend to ride before rollout procedure.

Also, you'll get the best results by getting an assistant to push you slowly over 100' calibration course - let them provide the push, you focus on steering a perfectly straight line. This is hard to do at very slow speed by yourself, while you're staring at your front rim and counting revolutions, plus aiming for the finish line paint mark. Probably would be easier if you have a painted line to follow, with calibration marks added.

If done properly, this method is accurate to 1/1000. On several occasions I compared my (cateye) reading to garmin gps, had agreement of ~0.05 mile over 40 mile course.

Or, 0.05/40 = 0.1%. Good enough.