Hi Dennis,
Yes. Fully concur with your examples of both 8" and 12.5" FL/FR.
So in order to get the same image scale (2000mm) as an 12.5" @ F/6.3, you need an 8" at F/10. Similarly a 40" requires FR of F/2 and a 5" requires a FR of F/15.75 to reach the image scale (2000mm) as both 8" and 12.5" scopes. Obviously the larger aperture instruments have a higher angular resolution (central obstructions aside).
Cheers
