I disagree. Like the secondary mirror (which behaves, for all practical purposes, as a field stop), the Paracorr field lens is also a field stop. In this respect, a larger secondary mirror is already being stopped even if the Paracorr lenses are removed. Using the Bartels calculator, the eyepiece field diameter represents the illuminated diameter represented in the off-axis illumination chart. If the Paracorr field lens is illuminated 100-percent, the internal Paracorr lenses are responsible for the illumination profile at the eyepiece.
OK, look, the new focal plane, with the Paracorr, is considerably farther out than the focal plane without the Paracorr.
Why wouldn't you use the new focal plane position as the intercept distance on the scope?
Because the Paracorr moves the focal plane out, so the illumination of a 40mm circle at the focal plane is now farther out.
But, the Paracorr barrel is inserted into the light cone of the scope inside the focal plane of the scope, and it's a long tube, with about 85mm of Paracorr inside the old focal plane of the scope.
(and maybe some drawtube, if it is longer than the Paracorr).
Let's assume, for the sake of argument, that you chose a secondary size as if the Paracorr were not there.
Then you insert the Paracorr, and you discover the edge of the front end of the Paracorr intrudes into the edge of field light path, which is already dimmed 30% by the secondary mirror.
If you chart the difference, you'll see the edge of the field is dimmed slightly, but not significantly--maybe another 10%.
Could you notice that vignetting? Probably not. And it wouldn't be particularly important if it only occurred in the lowest power eyepiece, either.
What happens if you increase the size of the secondary? It illuminates a larger circle, of course, and the Paracorr cuts off even more peripheral light (though it may be outside the eyepiece's field stop).
What happens if you shrink the secondary? It illuminates a smaller circle, and the Paracorr may cut off no peripheral light at all. Of course, the secondary not cuts off a lot more peripheral light than before.
So why in the world would you take the Paracorr into account when choosing a size for illumination? You wouldn't do it for a Barlow, and you wouldn't do it for a long drawtubed focuser, either.
You calculate the size of the secondary to illuminate the circle on the focal plane of the scope.
Now, you could think of the Paracorr as a part of the eyepiece, a part that reduces the 36.2mm field stop of the 21mm Ethos to 31.5mm.
Or, you could think of it as a part of the scope that increases the focal length and f/ratio by 1.15x.
Either way allows correct figuring for the magnification and the true field.
But replace the Paracorr with a 1.15X Barlow. Would you use the Barlow to calculate the size of the secondary?