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Tony,
The maximum differs for most reflectors vs refractors:
- for reflectors, 1.5X per mm of aperture;
- for refractors 2X per mm of aperture.
And yes you are reasonably correct in suggesting 200-250X is about the upper limit for most scopes if the seeing is average ("average" seeing being about 1" of arc).
With the right night, the altitude of the object above the horizon, and your location you can do better, of course; though for most amateurs these nights are about 1 or 2 a year.
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