I'm about to be guilty of being absolutely no help whatsoever (!), but really? You need to ask that on here? You roused my curiosity because I found myself having completely forgotten the theoretical formula, but a 20 second google search led me to the answer. Much quicker than waiting for replies on here as well, although paco's already given the answer for you.
Ok, ok, I won't be completely unhelpful, the formula is 1.23 multiplied by the square route of the altitude in feet.
