One of the primary reasons why it's impossible to have any absolute answers in photography is us. And I'm not just talking about personal taste but how our mind perceives tones. Take for example simultaneous contrast. You can define what the reflection density values should be, but as soon as they come together all jumbled up in a print, it can be a completely different ballgame. The attachment has a fun example of simultaneous contrast. The background is a gradient. On top is a bar of a single tone of 50% gray.