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A good rule of thumb for maximum useful magnification is to use an eyepiece with a FL in mm equal to your focal ratio. I've determined empirically , for my eyes at least, that in the fine seeing the first diffraction ring becomes detectable at this magnification and any more is essentially `empty' in that you are losing image brightness for no gain in resolution , and floaters in your eyes become much more problematic. A scope without tracking may need to use a little higher to exploit the seeing as the eye cannot grasp and mentally co-add image detail as well.
It happens that this rule also equates to a magnification equal to your aperture in mm or a 1mm exit pupil. If you are not seeing a clearly defined and occassionally recognisable airy pattern then you are allready in the realm of empty magnification.
Bright objects like the Moon, and Saturn with its sharp edges and little in surface detail seem to flout this rule at least on an aesthetic level, as there is so much light to play with.
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