f/stop timing and partial stop calculation
You might also note that in my article Test Exposures in Printing I provide a handy chart of exposure times in 1/4-stop intervals from 5 seconds to 160 seconds. All you have to do is print the chart and put it up next to your enlarger. I do note in the article that, if you are making exposures less than 30 seconds, 3 second test strips are almost as accurate and a lot less trouble. But if your exposures are above 30 seconds you may find that precise 1/4-stop test strips are more useful in finding the desired exposure quickly.
f/stop timing and partial stop calculation
Another way to do long exposure test strips is to give the whole strip a basic exposure then add smaller increments. For instance, if I judge the required exposure to be "somewhere around 25 seconds", I will expose the whole strip for 20 seconds, then add 3 second incremental exposures. This would give sections of: 20, 23, 26, 29, 32, and 35 seconds. For longer basic exposures, I make the increments larger; and smaller increments for shorter exposures. Maybe not as "arithmetically correct" as f-stop timing, but (IMO) a heck of a lot easier.
f/stop timing and partial stop calculation
Well, what an amazing discussion this has started.
With the help from the above answers, I have created an excel 2000 spreadsheet that I have carefully formatted for readability and printability that has a complete table of f/stop timing intervals from 4 seconds to 128 seconds. It gives you intervals in ? stops that can be used for creating custom test-strip/print series to your liking. Each increment also has a full listing of "plus" and "minus" times in ?, ?, ?, &1 stops to use for burning and dodging or any other adjustment to exposure that you want to do in the f/stop style.
I have this available to anyone for free in either excel 2000 form or as a .pdf (adobe acrobat) file. Both are ready for printing. (There is nothing commercial about any of this).
Please contact me at the address below if you would like me to send you one or the other. It's fun to be able to share information...
Scott