Three radii have been held common to ease construction problems. The on axis images are not as small as some of the previous designs but performance is fairly well balanced. Element thicknesses could be reduced by using extra elements but this means more light loss and complexity. The flint component closest to the eye is a little thin and delicate but helps to reduce steepness and diameters of all following elements markedly. If nothing else, this eyepiece would be a guaranteed centre of attention.
Curvature of field becomes the real limit but with an efl.
The best compromise must depend on the type of use, the type of eye, the contents of your optical junk box, and the depth of your pocket. Although some of this information might be irrelevant or impractical to your needs, I hope it sets the Magnum eyepiece in a clearer perspective. I know that this exercise has done that for me. Examination of the raytracing data may reveal apparent discrepencies. The size of the field lens, for example, is different in each case.