If you're like me, it's difficult to remember how each modifier changes the exposure, which makes guessing the correct exposure a matter of luck. Here's an easy way to get a quick, accurate estimate.
In this example, I have a small soft box that I sometimes use with my off-camera flash. I used my light meter to establish a base exposure, in this case my Canon 550EX set at full power at six feet away from the subject will give me an exposure of f-5.6 at 100 ISO. Knowing that, I can quickly estimate the exposure I'll need for different distances, apertures and ISO settings.
This does require a basic understanding of apertures and the inverse square law, but it's easy to learn and there are articles on both on this blog. For example, if I change the power to one half power, my aperture would change to f-4. If I changed the ISO to 200, the aperture would be f-8. If I moved the light to nine feet away, the aperture would be f-2.8. Easy.
When you're out shooting strobist-style, this can help you get the right exposure fast. Do you have any tricks you use? Tell us in the comments!