I scratched my head about this also when I first used ViewPoint a long time ago, but once I saw that we have an option to choose a 'Natural'
adjustment on the Perspective sub-pallette (you'll recognise it in my attachment) ... I took this as a big hint that Perspective corrections would always be more or less subjective.
Then when I saw the description of this 'Natural' Perspective option in the ViewPoint Help
, where it says "Natural: Applies a correction with the intensity pre-set to 75 for a more realistic rendering"
, I felt even more certain that ViewPoint did not (and probably could not) give me an absolute, or automatic, perspective adjustment for any of my photos.
I only do architectural photography occasionally, and only as a hobby, so I don't know what methods 'real' architectural photographers might employ to be sure of getting the 'best' perspective each time from their photos, but I'll bet that even they would agree that these adjustments are more of an 'art' than a 'science'!
I'd be very happy to be proved wrong of course