How far from the computer screen is it recommended to sit?

This page which turned up on a search for "ideal distance from computer monitor" states:

viewing distance - the monitor should be at a comfortable horizontal distance for viewing, which usually is around an arms length (sit back in your chair and raise your arm and your fingers should touch the screen). At this distance you should be able to see the viewing area of the monitor without making head movements. If text looks too small then either use a larger font or magnify the screen image in the software rather than sitting closer to the monitor.

Which isn't definitive, but does raise the good point about adjusting the screen resolution and/or font sizes rather than moving closer to (or further from) the screen.

I've asked the same question before. It appears the "one arm rule" to be in place. However I tend to disagree for larger displays like 20+". Wikipedia also has a nice article about calculating viewing angles, distances and field of view.

And my own adapted rule for screens above 14" is: distance = 1.5 * screen diagonal.