Place the valve stem at the very bottom of the wheel, as if it's intersecting the ground, and mark that spot. Roll one complete revolution, and mark that spot, and measure. That's the circumference. Enter that to start. Then ride a known distance and check your computer.
If your computer is reading less distance than actually traveled, then your entered circumference is larger than the tire actually is, creating fewer recorded revolutions than actually occurred. Adjust the entered circumference down by the % difference. If your route is 10 miles long and you recorded only 9 miles, then adjust your entered circumference 10% down and ride the route again.
If your computer is reading more distance than actually traveled, then your entered circumference is smaller than the tire actually is; adjust your entered circumference up by the % difference and re-check.