I always calculate for a 40" WOF plus buy more than I need for a given project. If it's for a focus fabric or background I'll overbuy as much as 1-2 yards. For borders/bindings than maybe .5-1 yd.
Even with this plan, I'm still waaaay short on fabric for my current project...sigh. Oh well, another trip to Lancaster in my relatively near term future, lol.