Well, the field stops are "effective" whenever there are any lenses in front of the focal plane, and that includes everything except for the Plossls and Panoptics. To answer your question, however, I suspect they did essentially what you said in a "virtual" design environment (to say a computer program), and set initial guess at the effective field stop diameter to be about 8*100/57.3, which would be about right for no angular magnification distortion, 8mm focal length, and 100 degrees of AFoV. The effective field stop diameter would be the physical field stop diameter adjusted for the effect of the eyepiece lenses between the telescope and the field stop. Consequently, this would be a calculated constraint for any particular lens configuration. There are lots of constraints to juggle, however, and so it is not surprising that the final value is allowed to deviate slightly, as it might be considerably more important to control something else a little more tightly and to ease up slightly on the idea of dead-on zero angular magnification distortion. The general tenor of this thread notwithstanding, there isn't much practical benefit to hitting that particular mark exactly.