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Well, pretty much already covered: select minimum and maximum useful magnifications for your scope and viewing habits (e.g. 50x and 300x) and the corresponding exit pupils, and minimum and maximum useful exit pupils for your eyes and conditions (e.g. 0.5mm and 6mm). Combine the two to get your upper and lower limits. Fill in between the limits using f/stops.
A couple of example calculations (all figures approximate):
10" SCT, F/L 2500mm, f/10: 300x >> 8mm eyepiece with exit pupil = 0.8mm. Result OK.
10" SCT, F/L 2500mm, f/10: 0.5mm exit pupil >> 5mm eyepiece with magnification 500x. Result unusable.
Depending on scope parameters, a good exit pupil size may give an unusable magnification, and vice versa.
For scopes with a central obstruction, there is a lower limit of magnification else the shadow of the obstruction becomes visible - I've seen ~35-40x quoted, but I generally stick to 50x.
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