I like to have accurate speed and distance figures, so I measure my own personal wheel setting. This how I do it:
1. On a quiet piece of smooth road or a long driveway, measure off a 528-foot distance.
2. Inflate your tire to your normal ride pressure.
3. Set the CA distance to 0.00 and the wheel size to 1515 mm.
4. Ride your bike over the 528-foot course. Turn around and ride back; continue riding until you have covered the 528-foot distance ten times.
5. Read the CA distance. You want it to be 1.00 miles, but it likely won't be that the first time you do the test.
6. Correct the wheel size based on the CA distance readout by dividing original wheel size by the CA distance over the 5280 feet ridden. This figure is your new wheel size (for example, 1515 divided by 1.04 miles = 1457 mm; or, 1515 divided by 0.96 miles = 1578 mm).
7. Reset the CA distance to 0.00 and repeat the test with the new wheel size. Your measured distance should be much closer to the desired 1.00 miles. Repeat until you are satisfied with the accuracy.
I use a full mile for the measurement to reduce the effects of small errors. You need to use the tire pressure that you will be normally riding with and you should be on the bike (your weight will influence the wheel diameter, especially at lower pressures). If you later ride with a lower tire pressure than what you tested with, your CA speed and distance will read higher that the true speed, and lower if you ride with a higher tire pressure.
For my recumbent with a 28-406 mm smooth front tire, my final wheel circumference at 120 psi turned out to be 1476 mm.