You need to measure the entrance pupil, the apparent size of the aperture through the front element, I hold the lens at arms length and measure with plastic calipers.
The f.l. is easy to measure, focus at infinity, mark the bed/rail. Focus at 1:1 (use a ruler and measure the image on the g.g.), mark the rail/bed. Subtract the second number from the first and you have your f.l.
Aperture is simply f.l. divided by entrance pupil diameter (expressed in the same units, of course)