Which brings us on to the second important factor, the size of your screen. Obviously the bigger your screen the further back you need to sit to make sure you can't see the pixels, although again the resolution will affect this distance. All TV screen sizes are given in terms of their diagonal measurement and you will need to know this so you can calculate the ideal viewing distance.
The graph above gives you a good idea of how the optimum seating position works, with distance on the vertical axis and screen size on the horizontal. So as you might expect, the lower the resolution the further you need sit away from the TV and the smaller the screen size. The area labelled 480p refers to standard definition, although that's the US TV standard and it would actually be 576p here in the UK. So based upon the graph you can immediately see that the seating distance should be between 2.4 and 6.1 metres with screen sizes ranging from 20 to 55 inches.
The next section on the graph equates to 720p, although you could also count the 1080p section as together the two would be considered high definition. Again the graph shows that viewing distances can range from 1 metre all the way out to 6 metres and screen sizes that go from 20 inches up to 100 inches. Finally there is the ultra high definition 4K section where you could sit as close as 0.5 metres from a 20-inch screen and 1.2 metres for a 100-inch screen. Of course these are just rough estimates but it gives you an idea of how optimum viewing distances is calculated.
However multiplying by a factor of 2.5 is more related to the optimum viewing distances recommended for a standard definition TVs and since high definition TVs have much higher resolution, you can in fact sit much closer. SMPTE (Society of Motion Picture and Television Engineers) recommends a viewing distance that is 1.6 times the diagonal width of the screen. This is generally regarded as idea for high definition TVs and so, using the same example, you would sit 80 inches or 6.6 feet (2.03m) from the screen.
THX however feel that the viewing distance should replicate the immersive cinematic experience as closely as possible. So they recommend a viewing distance that is 1.2 times the diagonal width of the screen. By sitting closer the screen fills more of your field of view and THX believe that high definition content has the resolution to handle these closer viewing distances. This means that using our 50-inch screen for a third time, the distance would be 66 inches or 5.5 feet (1.68m).
The approach taken by THX brings up another factor that can also be taken into account, that of viewing angle. This really just means how much the screen fills your field of view, the bigger the angle the larger the image. The viewing angle used by many manufacturers equates to 20%, with the SMPTE guidelines it's around 30% and for THX it's 40%, which they feel gives the most immersive experience. Whilst we're on the subject of viewing angles, clearly for the best experience you want to be sat directly in front of your TV and you don't want it too high - you shouldn't have to look up more than 15 degrees.
In fact the THX calculations would actually work as a general estimate of viewing distances for Ultra HD TVs, especially when compared to the other guidelines. In fact when it comes to ultra high definition, you will need to sit quite close to your TV to fully benefit from the higher resolution, especially with screen sizes below 65 inches. Of course ultimately these are purely guidelines and there is no right or wrong answer. In fact even the industry bodies involved can't agree on a single solution, so ultimately it should come down to personal preference.