There seems to be some contradiction between the calculations and Ole's post, which is the explanation that my understanding supports. These f/13, f/15, f/30's etc. don't make sense. The US (not United States, you Nacirema, but Uniform System) preceded the f/stop system we use today, phased out around 1909. I have 2 Goerz Dagors marked in these stops, and the way Ole has expressed it has worked for me perfectly in making 8x10 negatives since 1970. Since the Dagor was first released in I think 08, I know these lenses to be among the first of their type. Using the cross at f/16 has not once resulted in an error.
So there has to be some discrepancy, somewhere, either in the measurements, or they simply can't be US numbers. Which is it?