20/20 vision resolves one arcminute, and a degree holds 60 of them — so about 60 pixels per degree is the point where a pixel grid stops being visible. Past it, more resolution changes nothing you can see.
PPD depends on distance as much as on the panel, so sitting back doubles it just as surely as doubling the pixel count. It is why a 55-inch 1080p screen at 8 feet already reaches 67 PPD, and the 4K version of the same screen looks identical from that sofa.
20/20 is a normal, not a maximum: plenty of people resolve 20/15 and see structure past 60 PPD. Vernier acuity is finer still, which is why a misaligned line or unsmoothed text edge stays visible on a display the rule calls retina.
Estimate with the rule, then check it against the calculator that models it properly.
Open Screen Size & Viewing Distance →20/20 vision resolves one arcminute, and a degree holds 60 of them — so about 60 pixels per degree is the point where a pixel grid stops being visible. Past it, more resolution changes nothing you can see. PPD depends on distance as much as on the panel, so sitting back doubles it just as surely as doubling the pixel count.