If you're moving the flash independently of the camera, one option you might consider is moving the flash in f-stop increments. Moving it from 8 feet away from your subject to 11 feet, or 8 inches to 11 inches, or .8 meters to 1.1 meters would mean you get one stop less light from it. Moving it from 8 feet to 5.6 feet, or 8 inches to 5.6 inches, or .8 meters to .56 meters would mean you get a stop more light. In other words, move the flash in f-stop numbered distances relative to the subject and you'll know how many stops to adjust.

Lee