prolly be easiest with the shock off the bike. set the bike up on a stand and measure the distance between the upper and lower shock mounts. match that to the length of the shock eye to eye. then, measure the shock shaft that is exposed. that is the only distance/amount that the shock can compress. if you exposed shock shaft is 4" for instance, move the tire up until the distance between the upper and lower shock mounts is 4" less than before. then, just measure how far off the ground the tire it. total wheel travel...
hope that helps.