How far to sit from a screen — by viewing angle, checked against its resolution.
Sit 8.8 ft–6.5 ft from a 65" 16:9 screen
6.5 ft is as close as THX recommends and 8.8 ft is the conservative SMPTE distance; anywhere between the two fills a comfortable share of your field of view. The resolution holds up throughout — pixels only become resolvable closer than 4.2 ft, which is nearer than you would sit anyway.
What this cannot tell you
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Viewing Distance Calculator on RunTheTests — free browser-based device tools, no sign-up.