One operator of large-format cameras says just regard the focal length of your lens as an f stop and add any extension to it to get the next f stop. E.g., if you're putting a 28-mm extension tube behind a 50-mm lens, you're going from f/50 to f/78. These numbers look more like f stops if they're divided by ten: f/5 and f/7.8. So your 28-mm extension tube would call for about a one-stop increase in exposure time or maybe a tad more. This means that f/5.6 on your lens would be about equivalent to f/8. I haven't tried this, but it looks pretty good.