...and Learn to Take More Consistent Photographs
Just about anyone can tell the difference between two, four, eight feet and so on. Using this simple diagram has helped me to understand and quickly reference how to set up my flash to change the shadow effects of the image. It helps to avoid the ratio battle and confusion that comes from using flash and multiple flashes.
Using this method, which is the Inverse Square Law, any distance can be plugged in: feet, inches, yards meters, whatever your comfortable with or whatever the situation you find yourself in this will remain constant. Something so simple as moving the flash distance can change the effect of shadows in your image. Foreground or Background.
The Easier Setup...
What I've done with this chart, based on Speedlight's (or speedlite if your a Canon photographer) manual settings I've developed a very simple chart for what Power setting to put my flash on based on how far I am from my subject.