Good catch, Randy. Obviously, THX is using a 16:9 ratio to calculate the screen width, which is what really matters here. But I'm not sure that the given formula becomes invalid. There is a separate one for height (15 degrees).
Like Randy said, I think THX provides an optimum viewing distance for a screen that size of a little over 13 feet away (which might be fine for your room, eh?). People who sit appreciably CLOSER than that will have more than 40 degrees of horizontal image to process, which might be "too much".
I mean, seriously, 11 FEET diagonally? You guys make me sick.
