Hey Paul, I don't know if this is the sort of technical info you're after but an acquaintance of mine is into astronomy and mentions this site a bit
www.telescopeking.com
Specifically this page deals with calculating various factors such as magnification, focal length and so on
http://www.telescopeking.com/guides/...r-calculators/
Yeah it's for astronomy telescopes but I think the concepts are generally the same because as I understand it, maximum effective range is a factor of the image clarity at high magnification - and that all comes down to how good the lenses are, how they're aligned and so on meaning the same magnification on two different company's scopes may actually yield differing effective ranges.