Hi Glen, this is excellent stuff, congratulations!
However, your formula should be expessed as [9.1 + 5logD] x 85%. Took me a while to work this out, as I punched in my aperture and got a WML of mag 11.8, which was obviously too high (more like my threshhold, although I have seen some mag 12 objects). 10.5 is a better figure, but still maybe a touch high. Perhaps the formula might fall down a little with small apertures - would like to know your opinion on this. Certainly, no 2" telescope would have a WML in excess of mag 9.1, you'd think.
Cheers -
Rob