Scientific exactness is not required for good round numbers and exposures. Measure from the center of the lens between the elements to the ground glass when it's focused at infinity. Hold a ruler up to the front and measure the narrowest aperture (usually a bit smaller than the front light). Divide. That's it. ie. you measure 12 inches at infinity and it has a 2 inch widest aperture, it's a 12" f6 lens. I do this all the time with these antiques and get excellent exposures. Don't do the flashlight through the lens circle on the wall bit. You're not at infinity doing that and it's meaningless unless you're an ebay seller. All of a sudden you have a 10" lens to sell instead of a 6 inch one. :rolleyes:
Oops, sorry Jason. We were both posting at the same time.