IF: ISO=100, light is 6ft from your subject and flash is @ full power
Then: a good starting point is around f/22.
Several things will affect how much light you are getting in your exposure. ISO, Aperture, flash output, modifier (octabox), and distance to your subject. Shutter speed, not so much, as your flash is faster than sync speed.
The good thing is, most of this is predictable with math. ISO is 2x or .5x more (or less) light for each full stop change. Same with aperture and flash output. Assuming the modifier stays the same, there is no change to worry with. Distance seems hard to predict, but is not really. If the light is 2' from your subject, then you move it to 2.8', you lose one stop. Then to 4', another stop. 5.6', another stop. 8', another stop. 11', another stop. See something familiar?
Each time you move your light 1.4x further away, you lose one stop. Move it in by .7, you gain a stop.
All of this is fine and dandy, if you know where to begin (and take teh time to measure). To find your starting point, you can trial and error or use an incident meter (the Cyber Commander has a very good one, at a very reasonable cost, plus some). Ambient light meter readings (like those your camera meter provide) are worthless, as you will be adding more light than it can read. Theoretically, if you use an 18% gray card, filling the frame, and placed where your subject is, a properly exposed photo will generate a spike in the middle of your histogram (if you wish to not use a meter). 3/4 to the right is about 1 stop, far right is about 2 stops over exposed. Inverse is true with the left. Your mileage may vary, depending on camera.
Keep in mind, the above is a starting point based on theory. It assumes you want a "proper" exposure, and not a higher or lower key image. An external meter will allow you to measure ratios, measure light fall off, and is typically a faster way to work.
If you take similar/identical portraits (ala school photography), once you havea formula that works, i have seen photographers use a piece of string attached to a light stand to measure distance. Same distance, same light and settings, same ISO, same modifier, same aperture, same picture.
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