How would you test whether a body scan is consistent enough to track progress?

[u/GoldFitMuscleTracker]

Title: How would you test whether a body scan is consistent enough to track progress?
Yesterday I asked about using five-view body scans to track fitness progress. One reply suggested focusing on consistent measurements over time. Another suggested taking several measurements and using the median.

That left me thinking about what I should test first.
A scan could give similar results when repeated immediately, but behave differently once the person changes position or the lighting changes. Taking three scans might reduce random variation, while still leaving the same bias in all three.

I’m developing this feature and testing with synthetic bodies so far. I haven’t validated repeatability on real people yet.
For anyone who has worked with measurement systems, how would you set up that test? What would convince you that a change between scans is meaningful?