It you want to increase the accuracy of the computer, try doing a roll-out. You do this by measuring the actual distance traveled by your wheel. Mark a spot on the pavement. Position the valvestem of the wheel with the computer sensor on the mark. Now with your weight on the bike. go forward say 5 full revolutions of the tire. Mark this new spot and measure the distance traveled. divide by 5 (the # of revolutions), now convert this measurement to millimeters. This is a more accurate number to put into the computer.
I found the number in the charts to be pretty close, but this will allow for tire construction and the combined weight of you and the bike.