When dealing with these formulas, people often get confused on the viewing angle and get far too locked into the numbers. In my personal experience, the second formula is the one that works out to be the most accurate. However, if you are regularly using sources that support 1080p, such as a Blu-Ray or true 1080p PS3 games, then the third formula is usually the one to go with, which is the recommendation of THX. I can't truly argue with THX that the formula works, but one thing that consumers need to worry about is picking something that ends up being too big for their viewing distance, which really gets into the meat of this article: Pixelation.