I found a quote by Roger Deakins ASC, BSC regarding normal lenses for 35mm: Technically speaking a 50mm lens is considered 'normal'. The calculation is made in regard to the optimal viewing distance in a theatre and 'normal' meaning there is no distortion of view for the observer. With a the larger 65mm format negative it might be technically correct that a 110mm lens should be considered 'normal'.
For me 'normal' is something between a 40mm and a 32mm, when working with a flat 35mm negative format as opposed to anamorphic, A 50mm appears long to me.
The definition is an aesthetic and not mathematical one you are correct the 7.5 is the standard normal focal length but for some reason the viewing distance matters more to cinematographers than to still photographers.
This alone would make for an interesting topic. Normal focal length: viewing distance vs math. Or Choosing a lens based on the planed viewing distance.