Angle of View - Aspect Ratio -
Focal Length
Whoever deals with different film or sensor formats, will probably align his expectations, which angle of view a particular focal length will yield, to "equivalent 35mm focal lengths", at least I do. I however know that not everybody does, my father always thought of a 50mm lens as a wide-angle, since he was thinking in (Rollei) 6x6 terms.
To compare different focal lengths on different formats, the diagonal angle of view is usually looked at. This sufficient, as long as the aspect ratio of the formats is about equal.
But even if 35mm and 6x6 are compared, this shows its weakness - a 75mm focal length is (if the diagonal angle of view is considered) equivalent to a 40mm lens on 35mm. But horizontally the 75mm does not reach the angle of view of the 40mm - and vertically it exceeds it. And if panorama formats come into play, this becomes a useless comparison.
So, if aspect ratios differ, it makes sense to either compare horizontal or vertical angel of view, but not diagonal!
This is not rocket science, and there are even "Tools" on the internet. I however use a table, that might also be of use to others (sorry for the German descriptions).
While said 75mm lens has a diagonal angle of view of 21.4° in front of a APS-C sized format and a 35mm equivalent focal length of 114.4mm, things become difficult if the lens sits in front of 6x6 or 6x12 format.
Here is the table: