[John]

I turn until my left shoulder touches my chin. At that point, I'm as far back as I need to be for whatever club I'm using. If I'm swinging a wedge, then turning back until my shoulder touches my chin produces a 3/4 swing. With a driver, the same turn results in a full swing which is generally parallel at the top.
Doing it this way eliminates any need to think about anything other than touching my chin with my shoulder. I don't have to worry about where my hands are or how far my arms are from my body and so on. It's a very simple thing to remember and it will always produce the swing that's necessary for the club I'm using.