“Scale Speed” is apparently a term that invites differing interpretations.
If one is talking about the impression – that ‘looks’ realistic – then one should probably start with the mathematical answer (divide the full scale speed by the scale factor – a very simple computation). However, the speed that ‘looks’ right might vary from this simple answer, depending on the observer, environment, etc. Nonetheless, the math answer is very straightforward and a very good starting point.
If one is talking about aerodynamics (Reynold’s numbers, wing loading, etc.) then we are off into the weeds. Nothing wrong with that but it is not the same thing as a purely subjective answer.
One person here tried to introduce the totally obscure idea of ‘scale time’, then excoriates another person for talking about introducing ‘wing loading’. Really? At least some arguments are still based on some science, not science fiction.
Finally, to the original poster, 500 mph is NOT 4000 ft/second (which would be over 2700mph).