Get a piece of that "memory" foam that doesn't spring right back out after you sit on it. About 1/4" thick. Sit on it on a hard surface, like a bench or wooden chair/stool. Get up quickly, and measure the distance between each of the round depressions you left, center to center. You could always go to a Specialized dealer and have them do it with the "Ass-O-Meter". Convert your mearuement to millimetres.