The f stop value of a lens is defined by the focal length of the lens divided by the diameter of the objective lens (front element). so a 300mm lens with a 100mm front element has an f stop value of f2.8. That's how it works.I'm not sure you are correct. By this formula, the new 24-70LII has an f stop of about 24/64 = 0.375. But its a 2.8 lens. While the approximation approaches this as the focal length gets longer and the angle of view becomes narrower (telephotos and telescopes), it is not true for all focal lengths, and especially the wide angles.
My understanding is the f stop is calculated from the focal length divided by the apparent iris aperture via the optical formula of the lens (which includes all the magnifications of the optics).
I may be wrong, so I'd be happy to be corrected.