I'm not sure I have the words right. Anyway, I think I read somewhere that the average viewer can watch a screen with a maximum off center horizontal axis of 30 degrees. That is, the total horizontal viewing "angle" is 60 degrees. Similarly, the vertical off-center axis is 15 degrees.
If I assume 30 degrees off-axis is a horizontal maximum, it forms a triangle. What is the formula, given the distance from the screen, of how wide 1/2 the screen can be? I'm trying to get an estimate of a reasonable 16x9 sized screen given 10 feet distance.