There is no "average". I think the important thing is a clean back. Knots on the belly you can live with, a few pins coming cleanly through the back is ok, but you don't want any nasty discontinuities in the back on a heavy bow. I start getting twitchy over 90#
If you can get a stave where the back needs nothing doing to it at all and it still has the bark on it you are on to a winner, just carefully remove the outer bark, leave the inner stuff, it will pop off in it's own good time and it offers some protection against the inevitable bumps you get when working a very long stave.
Length is a great de-stresser of bows, so always go longer than you think, it's easy to take an inch off each end but difficult to piece together an exploded bow.
Don't get suckered into the high arched D of the Victorian longbow most of the Mary Rose Warbows looked more like an 'inflated square' cross section or almost round.
the other thing is tillering a heavy bow feels very differnt to a lighter bow, you have to be pulling serious weight right from the start and there is no such thing as floor tillering! If you can flex it on the floor, it's probably too weak already!
Del
(if all else fails wear a hard hat and safety glasses
)
PS. Getting a heavy bow braced for the first time is a nightmare, you absolutely must have a decent non stretch string. A continuous loop string with a string shortening ring (or such like) is best (IMO), a bowyers knot can just slip at high weights.
