Excellent question Alex and one quite on my mind, assuming we ever get transparency here again.
I have tried using the scales in Brendan Gazzard's article on IIS
"How to determine seeing and transparency" but increasingly find this system doesn't work for me. In regards to transparency, my skies are usually at least 6 if there is no obvious cloud cover, but recently high level moisture has meant that observing fainter objects is quite disappointing as galaxies float in murk. So this scale seems insufficiently sensitive for a dark sky. The scale for seeing works a little better, but it would seem to me that one based on the highest magnification that can used with stable images might be better.
I look forward to some useful answers!