I guess it depends on your printing method, but personally, unless somehow forced to, I would not use an f stop that would give me a 5.6 second exposure. Like I said before, I prefer an exposure greater than 20 seconds. If you do it that way, you can reduce some error, since you could start your first strip at 16 seconds and work up from there. If they all come out too dark, reduce the aperture to get the exposure in the window you want.