That is the point though. There is no "believable tolerance". According to our crafters here, there is just too much variance between pulls for there to be anything "uniform".
It is impossible for Thomas, or anyone, to show the differences. Aside from GINO's pieces and Thomas' SL, we are pretty much swinging at air. They would need access to a screen used piece, which even then, leaves us with the problem of no two pieces are the same from the same mold.
Well we do the best we can. The crafters here are basing their opinions on what? TK helmets? Boba fett helmets? You can't compare those to fiberglass Darth Vader helmets. There is surprising consistency and accuracy in measurement among authentic masks/helmets. The SL, the TD, the DJ, the MP (the ESB movie poster helmet) and the TM. Each from very different times, and very different sources far removed from each other and yet they share surprisingly similar dimensions....not exact of course but
very close and they are basically in the same size class with each other.
The next group down would be the VP ANH and the 20th Century, and the GH ANH. Below that is the size class that includes derivatives of the 20th Century, such as the JB, SPFX, VM and DP Deluxe/CA line. The ROTJ also are in that category. But as you get to that size class the diversity is much greater and so harder to pin down the source.
And there are ways of "calibrating" scale in terms of measurements of a particular casting. For example, I have 3M mounting tabs from 1979. They are consistent among each other in dimensions to high precision. I can compare them to the cast tabs on castings such as the TM, TD and 20th Century as a reference to something "original". It takes precision and accuracy, and a LOT of measuring to obtain a dataset that is reliable. For some castings I might have measurements from more than one example of that same source. I can't just compare something like five or six castings and come up with a trend, I need at least 30 or more (I have 35) and even then I could use more.
If someone else took the time to compare castings in that way, they should find the same trend. Is it just generational? Or is it based on the true lineage of the helmets? It is a guide and I use it in conjunction with examination of related details.