I don't believe there is a hard-and-fast rule here. Everyone has a different point where the clubhead naturally reaches the bottom of it's arc in the swing. Find out where that point is for your irons and fairway woods and position the ball about 1" behind it. In theory you should contact the ball before the ground and achieve a good strike. When it come to tee-shots, I think it's matter of trial-and-error. Experiment on the range.