The Thing Itself
Join Date: Jul 2008
Location: Boston, Massachusetts
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Just do the roll-out as mechBgon advised; don't worry about computing it, as you're likely going to be off in the inital measurement, and don't worry about the instructions as they're not going to take into account your exact tire size + flattening due to weight.
A small dab of paint (or whatever is of similar viscosity, really) on your tire will make a good pair of marks on the ground, the distance between which you can then measure.