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My original advice was based on the idea that a particular scope should start out with a 1X/2X/3X magnification scheme, where X= a particular number based on the aperture. For instance, on an 8", X=50, on a 12.5", X=70, and on an 18", X=80.
That's because the larger scope can use higher magnifications and for the same f/ratio, larger scopes have higher magnifications at the same exit pupil.
Is that all the eyepieces you would need? Probably not as seeing conditions are rarely the same everywhere (people under very steady skies tend to use higher magnifications) and you might find yourself wanting a magnification in between two that are derived this way.
Such a recommendation flies in the face of exit pupil considerations.
Take the 18" f/4.5 scope with a Paracorr. The effective f/ratio is f/5.18,
so a 2mm exit pupil would be a 10.4mm eyepiece = 228X. Most places, that would be too high a magnification for a mid-power eyepiece, even though it is a mid-sized exit pupil.
so though the exit pupil idea seems to work well for small scopes, it doesn't seem to work very well on large scopes, which is the reason I came up with a 1X/2X/3X idea for a basic set of eyepieces to build a collection around.
In my scope, that was 70x, 140x, 210x, or 26mm, 13mm, 9mm and I have built around that idea by splitting the 9mm into an 8 and 10mm and going beyond in the high end to 6 and 4.7mm.
But the idea of a basic set of 3 around which you can build a collection is not a bad one.
And if you choose to use exit pupils to do so, that's fine, but just remember it doesn't work very well in big scopes. I have a friend with a 28" f/4.2 scope. With a Paracorr, his focal ratio is f/4.8. A 9.6mm eyepiece would yield 358X. Where he observes, he can use that magnification fairly often, but not much higher. But that's a 2mm exit pupil, which is mid-power by exit pupil standards.
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