Sorry I’m late to this party. I’m currently in Israel up near the border of Lebanon at the ancient city of Dan.
The measurements I give are accurate to multiple digits and i guarantee them to be accurate so it MUST be something else. If your micrometer matches the SEL and the diameters are off, the only possible thing that would affect the calculation is that there is an entry in the Spacers field. If you have an entry there, it would cause your diameter to be less than the calculated diameter.
If you have an entry there but are making a painted segment or a tornado bowl, it won’t show you the vertical spacer because you would never make a bowl like that, but it may still affect the diameter. It shouldn’t, but I don’t have the ability right now to check that. On the new release, I will either not show you the Spacers field unless it is a feature ring, or I’ll go ahead and show you the spacer regardless of the type.