If it's a 1080p set, people will tell you (literally) that you should be about 6 feet from a 50" set. More like 8 - 9 feet for a 720 set. Most of these distance calculators are if you are going to be using the set for a theatre, not if you're going to be watching the evening news while eating dinner.

I sit 10 - 11' from a 50" for everyday viewing, and 14 - 15' feet from a 124" screen in the theatre.