Wheel centerline to inside mounting face of the sprocket.
So, I tried the measurement on my 1977 CB550F. Not so easy with the bike assembled and chain on the sprocket.
I measured between the inner flanges on the hub where the spokes mount. 50 mm. I expect the rim center will be at the 25mm point. I didn't verify actual tire center line, but assume it is same a flange midpoint using spokes of same length side to side.
Then, I measured from the sprocket mount side to the inner sprocket flange (trying to eliminate parallax error since they are on different planes). 60mm (I doubt this is hub center, fyi.)
That would make it 85mm from wheel center line to sprocket flange mount surface.
I didn't check my other Fs or Ks.
Cheers,