This is fairly accurate. 20mp will give you a lot of room in print sizes. Normal perceived quality only starts to deteriorate at around 60%, though consistency is a factor. As someone has already noted, you shouldn't go and mix different resolutions in same display form.
What is missing from the calculator is angle of view data. A lot of research went into it in previous decade, primarily for film use, but it found place in galleries and museums as well. Various lens magnifications are not irrelevant for printed or displayed photographs. Standard view of forty to fifty degrees on 35mm film, will be suitable for half a meter viewing distance at 40x50cm print. Viewer being closer and further away will constrain and expand natural view. Not a flaw, but for the time's sake we'll talk about natural viewing distances. The same print size, same viewing distance, with only change being a use of ultra wide lens of 20mm, will produce different viewing experience. To get normal viewing field of view at same distance as was previously noted, print would need to be 50x75cm. Psychologically it will still feel constrained for the viewer, and made him back from display, but it will still be a normal viewing angle.