I'll give it a try from 5 ft...didn't think I had to be so close...maybe you opened a new world for me...LOL
Rob, just for experimentation: If your 3D HDTV plasma screen is 65" (or even 60" for that matter) in diagonal, sit 65" from it.
Then put a 3D Blu-ray movie like
'Avatar', or any Disney PIXAR 3D animation flick with a 1.78:1 or 1.85:1 screen size aspect ratio.
If you feel it's too revealing, back up a little bit, but not too much; stay within six feet (72") maximum from it.
With a passive 3D display (LCD LED) you cannot do that (sitting that close from the screen), you'll see onscreen horizontal lines, and you have half the plasma's active 3D vertical resolution, which gives you decreased vertical detail and decreased smoothness, more grainy look than active 3D.
<<<>>> For optimal 3D viewing performance all the important parameters should be set properly; contrast, brightness, color, tint, sharpness, gamma, cell light, the proper picture mode (with the less discoloration; most natural colors), and color tone.
All the other stuff (enhancement this and that) is mainly set to off, or zero.
Again, this is for 3D (and 2D as well) experimentation purpose.
But in real life, for regular videophiles (less perfectionist and exploratory ones), sitting six to seven feet from a 65" class HD display is a good effective and performing standard distance.
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Viewing Distance Calculator - Best Guide
I personally prefer above THX 36° viewing angle recommendation. ...Bob's own 40° field of view.
...For serious critical viewing, and full 'immersive' visual impact