You want to say "the minimum that is useful", because you want a resolution at least equal with that, to not see the pixel structure. A 27" monitor has a heigh

You want to say "the minimum that is useful", because you want a resolution at least equal with that, to not see the pixel structure. A 27" monitor has a height around 17", i.e. about 43 cm, and for watching a movie or anything else where you look at the screen as a whole the recommended viewing distance is twice the screen height, i.e. about 86 cm. At this distance, the resolution needed to match the human vision is provided by a height of slightly less than 3000 pixels by this study, but by about 3300 pixels by older studies. In these conditions you are right, the minimum acceptable resolution is around 200 ppi. This means that a 27 inch 5k monitor, with a resolution of 2880 by 5120 pixels, when viewed from a distance twice its height, i.e. about 86 cm (34 inch), provides a resolution close, but slightly less than that of typical human vision. (That viewing distance that is double the height corresponds to the viewing angle of camera lenses with normal focal length, which has been based on studies about the maximum viewing angles where humans are able to perceive a correct perspective when looking at an image as a whole.) However, when not watching movies, but working with text documents, you normally stay closer to the monitor than that, so even a 5k monitor is not good enough (but an 8k monitor may be enough, so that might be the final monitor resolution, beyond which an increase is useless).