guide number = f/stop x flash to subject distance
flash to subject distance = guide number / f/stop
f/stop = guide number / flash to subject distance
These three equations are all that you need to know in order to manually calculate flash exposure.
For instance the LumoPro LP160 has a guide number of 140 @ ISO 100. So, if I place the flash 10 feet away from my subject the equation would look like this:
f/stop = 140(guide number) / 10 (flash to subject distance)
If you solve the equation you would set the aperture to f/14 to have the correct exposure.
You can plug in any two variables to get the correct flash exposures needed. Just keep in mind that these formulas are a good starting point. The end decision of exposure is up to you.
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