Millimeters? You put a tape measure on the ground, start the tire with the stem at the beginning of the tape measure, and roll the tire forward until you've made exactly 1 rotation. Where the stem ends up is how many millimeters you enter.

Or you could have the bike store install it, that's what I usually do as they're going at having ties and running the cable so it does not get snagged on anything later breaking the unit.