And my preference, Rule #2: Measure the width of your TV screen. Optimum viewing distance for TV is 3-6 times this width and 1.5-3 times for DVD/HDTV. Since you have a nice overlap at 3x you can use it for the best all around viewing distance/TV size. Now since regular TV is 4:3 ratio you can use 3,4,5 triangle math and come up with the one simple rule: TV size (inches diagonal) divided by 5 gives you the optimum viewing distance in feet.
Now remember this is the optimum setting for you Home Theatre/Main TV. So while a 70″ projector image is perfect for a 14′ distance to the main couch in the living room… putting a 55″ screen for the 11′ across the bedroom is a tad overkill.