I'd love to know how people come up this these viewing distances. They never explain and, of course, they never explain what criterion is being used to define "optimal." My calculations are very differ. I would base distance on 2 criteria, visual resolution and size of the focal vision area.
An HDTV is 1920 pixels wide. Maximum visual resolution is 60 cycles/degree. 1920/60 gives 32 cycles. So you want the screen to be slightly less than 32 degrees wide. A 50" diagonal TV is 43.5 inches wide. If you sit at 72 inches, the the screen size is arctan(43.5/72)*180/pi= 31 degrees. This is just about right. Further, focal vision, the part you use for inspecting the world is the central 30 degrees, so the screen is also optimal size by this criterion too.
6 feet is the optimal distance.
There are two caveats. People with long resting accommodation and vergance distances may have eye strain problems. Also, you might seen vernier offsets which require much higher acuity, in some images like cartoons.