For my rear derailer, I can twist the barrel adjuster until I see the pulleys line up directly under the gear I intend the bike to be in. So, if I want the bike in the 3rd smallest gear, I turn the barrel adjuster whichever way it takes until I have eyeballed the pulleys directly under that gear. Generally, when it works for one gear, it works for them all. I think for my derailer, turning the adjuster to the left moves the pulleys toward the frame, but I could be wrong. Anwyay, it is very nice visual feedback on what happens when I turn the knob.

edit: this is a barrel adjuster on the derailer, not the one on the shifter which I do find somewhat baffling...