Mel Bartel's terrific "Diagonal Off-Axis Illumination Calculator" calculates the illuminated field for a given EP field stop, using the mirror diameter, focal length, intercept distance and diagonal diameter.

But what is the proper way to incorporate the effect of a coma corrector that magnifies 6% or 15%? Usually people think of it as "increasing the focal ratio", as if it made the focal length greater, but really it doesn't, it is a modification of the behavior of the glass assembly at the focuser. So my thinking is that it is the field stop of the EP you are using that should be reduced to model the effect correctly.

Am I right about this?