Good info in Stan's book. Nice article here: http://www.brewlikeamonk.com/?p=113
Doesn't directly answer your question, but just take a look at that FG.
Thanks for the link. 90% attenuation on an 1.081 OG brew (1.081 to 1.008) would probably require a good amount of sugar, but I think I could get that with 10-15% sugar and the right mash. Plus the right yeast and good ferm conditions of course. Maybe I'm overly optimistic, I don't typically make a lot of 1.080 beers.