But if you're reeeeally lazy, set your f/stop to one inch and measure the bellows extension in inches. Or set it to one centimeter and measure the bellows extension in centimeters. Then you're just dividing by one; you bellows extension is your f/stop.
See this lens currently on ebay, #330714904774:
Notice it's got a big white mark at about f/11? Let's check my old Velostigmat and see what that measures to be...
Hmmm... about an inch. (Remember, there can be a little slop in the iris accuracy, just like on shutter times.) There once was an old photographer who had the option of measuring his bellows extension, and that was his f/stop, period. 18 inches of extension? f/18. 14 inches? f/14. 26 inches? f/26.
I can't swear that this is what was going on, but I've seen a few shutters marked this way, and it's always one inch, sometimes also at half an inch (easy math, cut your extension in half!), or one centimeter. If you aren't fussy about your taking aperture, it can make life easy.
Today's photo-tip for lazy math-phobes!