But for a display, you want to be able to have diagonal (or curved) lines at 1/150 inch apart, with pixellation artifacts that are small relative to those lines.
That probably translates to something like 450ppi (line width of 3px, so the artifacts should probably be about 1/3 of the line size).
His calculation gives 530 ppi at 20 inches.
Explanation: "The eye is not a single frame snapshot camera. It is more like a video stream. The eye moves rapidly in small angular amounts and continually updates the image in one's brain to "paint" the detail. We also have two eyes, and our brains combine the signals to increase the resolution further. We also typically move our eyes around the scene to gather more information. Because of these factors, the eye plus brain assembles a higher resolution image than possible with the number of photoreceptors in the retina."
Aliasing is another big deal. Jaggies are very visible until you get into quite high pixel densities.
Beyond this limit, I suppose there are people with 20/1 vision or something, but they're pretty rare.