Get on a long smooth level stretch of pavement, put the velve stem straight down, mark that spot, roll the wheel perfectly straight with a little load on the front tire for 3 revolutions or so, measure distance (in millimeters) of 3 (or what ever number of) revolutions. Divide by 3 (or # of revolutions) to find the number you need to put in your computer. I have only seen computers use millimetres for "roll out" entry, which will always be more accurate than a generic one size fits all number in a manual. A helper will make this task much easier.