Evan, you’re correct that it isn’t an exact 2 mm measurement. But it’s close enough that any difference will not be extremely meaningful. Your best bet to provide the most precision is to take the measurements consistently at the longest distance possible (outside end of the spindle).
When the odds are that your wheels may not be “exactly” straight ahead, when the tie rod sees a tiny bit of adjustment when the ends are tightened down, when the kart flexes on the ground, etc. All of these things make having an exact measurement next to impossible.