In addition to considering the probability of a landout, you have to take into consideration the consequences of one. If your final glide is over a landscape of neatly mowed large fields, the consequences of a landout are trivial and you can push closer to the theoretical limits. As Cliff says, many western sites have very poor options and carrying extra height, perhaps even more than mathematically indicated, might be wise.

I once got very very low over Zion National Park in Utah and have carried higher safety margins ever since!

Mike