Let's say your flash has a 120GN at the film speed you're using and the light will travel from the flash to a reflector umbrella where it is reflected to your subject. From the flash head to the umbrella is 1 foot, say, and from the umbrella to the subject is 9 feet, totaling 10 feet. 120, the guide number, divided by 10, the distance the light travels, is 12. So an aperture setting of 12 would be most appropriate for optimal depth of field.

In all likelihood your lens doesn't have an f/stop of 12, so you would use the one closest, which would be 11. Many lenses will have half or one-third stops as well, so to play it safe, you would want to bracket your shot, taking one at 11, one at a half or third past 11, and one at two thirds or the next number. You must remember though, you still have to use a shutter speed that will sync with the flash, generally 1/125th, 1/60th, or slower depending on your camera. If the shutter is not open during the entire duration of the flash, the subject will not be properly illuminated.

And over here we have...
Here comes the tricky part, figuring the guide number when using more than one flash. Honestly though, if you're using more than one flash, you really need to invest in a flash meter.

If you're using a second fill-flash that is further away than the main key flash, or is diffused, then the guide number for the main flash can stay the same. But if you're using two equal lights an equal distance from the subject, you multiply the guide number for a single flash by 1.4 to get the guide number for the shot. I'm not going to get into what to do with any more than two lights, because if you have more than that you most definitely should be using a flash meter.

For the most part, when using two flashes of different strength, at different distances, you can use a reasonably simple method to figure the guide number. First, calculate the aperture setting for each light by itself. If they come out to the same aperture, then use one stop smaller. If they differ by half a stop to one and a half stops, then use half a stop smaller than the smaller of the two apertures. If the two aperture settings differ by two stops or more, then use the smaller of the two apertures and disregard the larger one.

So if we take our first example with a flash that has a 120GN at 10 feet, we came up with an aperture of f/11. By adding a fill flash with a 100GN at 12 feet, we get 8.3, or an f/stop of 8. According the formula above we would use half a stop smaller than f/11. Now, my lens only has full stops, and there is only one stop from f/11 to 16, so I would shoot one at f/11, and one at f/16 then choose the best print from the two.

A monkey in the wrench
Here's where it all goes horribly, horribly wrong. In preparing this piece, I took some shots to provide examples of photos taken by the guide number system as compared to metered photos. I must admit that I was puzzled as to why the f/stops were dramatically different between the metered shots and those calculated using the guide number system.