A lot of the factors also bring in assumed viewing distance and size of the photograph being viewed. I just do not get wrapped up in terms like "normal lens" for viewing perspective. If you want to make the subject look closer, use a longer lens. Want to get more into the picture, use a shorter focal length.
Kingslake, "Lenses in Photography", page 7, 1951edition: "The 'Normal' field is usually such that the diagonal of the negative is equal to the focal length of the taking lens. ... As mentioned, this angle is rather wider than the eye can cover at a glance, but in practice we tend to view most photographs from a point well beyond the center of the perspective, and we unconsciously scan a print with our eyes."
What does that mean? It's easy to measure the diagonal of the negative and easy to measure the focal length of the lens. When the two are equal, you get
Focal-Length/ Diagonal equals 1. That's Normalized.