You also asked about viewing distance and angle. It's hard to say in abstract or statistically. But think of it this way. Suppose you had a group of people in your living room and you wanted to show them your prints. You set up the desk light, bring in the prints, and you'd want them to be viewed at a distance from the lamp whereby they are fully lit, but not too bright and not to dim - comfortable on the eyes. Whatever that distance, it is the correct distance. For beam angle, it matters more for gloss and luster papers. You don't want them viewing hot spots and glare, so the correct viewing angle is the one that shows the image to its fullest, absent as much of this spurious reflectance as possible. Whatever viewing angle that requires is the correct one.
