Join Date: Oct 2002
Location: Medway, MA
Bikes: 2011 Lynskey Sportive, 1988 Cannondale SM400
The only truly accurate way is to do a rollout. Put something on the tire of the wheel with the sensor that will transfer a mark to the ground. Then ride the bike with the tires inflated to your preferred inflation pressure. Measure the distance between two marks, and that is your number (in mm).
STAR 3 Spinning Instructor
2011 Lynskey Sportive (Shimano Ultegra 10s)
1988 Cannondale SM400 (Suntour XC-M 7s)