If the tire isn't in the list, take your tire and align the stem to some mark on your driveway or sidewalk or other flat surface. Roll the tire one complete revolution until the stem is upright. Using a tape measure (preferably one that also has centimeters) measure the distance from the mark where you started to the stem and multiply that by 100 (if measured in centimeters). So, if your measurement is 204 centimeters, your setting in the computer should be 2040. If you only have a tape measure that is in Imperial measurements only, you have to convert the measurement to metric first. Don't concern yourself with the exact parts of an inch. You can convert them to the nearest 1/4" and that is .25, .5 or .75 which you can use in the metric conversion. The difference in the 1/4" rounding isn't going to make a difference in the speed or mileage on the computer. Note: if you change tires where the tire is a different height (from bead to thread) than the original, you will have to do this again.
HCFR Cycling Team
Ride Safe...Ride Hard...Ride Daily
In addition to Nigel's excellent post, pump your tires to your preferred riding pressure, and sit on the bike while you do the wheel rotation.
Getting precise isn't too necessary unless you do really long rides (40 miles plus). I go on long rides with others, and our ending trip odometers can be 0-3 miles off comparing each.