AFAIK, the spacing required is between TSFLAT2 lens assembly and the field stop of the eyepiece. The field stop is where the final image is formed. Otherwise, how could the field stop look sharp when you view through the eyepiece?
So to measure the correct spacing for the TSFLAT2, you need to think about the distance between the TSFLAT2 and the field stop of the eyepiece. Any other considerations can confuse the issue and result in an incorrect spacing.
Tele Vue provides the distance between field stop and shoulder of the eyepiece, which is helpful. (Eyepiece dimension "F" in the table.)
But in practice, the easiest thing to do is just perform field experiments. Try each eyepiece with TSFLAT2 screwed onto the diagonal barrel, and then with TSFLAT2 screwed directly onto the diagonal without the diagonal barrel.
So far I haven't had to add barrel extensions to the diagonal barrel to increase the spacing in order to achieve a flat field.
On the other hand, if the distance is still too long without the diagonal barrel, you can't get any closer any way.
Edited by Sarkikos, 09 November 2020 - 07:53 AM.