Well, my experience says simply you want 1-2 meters plus to avoid casting ugly flash shadows on the background. As long as subject to background distance is taller than subject, and as long as you put flash high enough to direct shadows downward, you are fine. Also, the camera-subject distance should be longer than subject-background distance, and the subject-background should be long enough for natural "separation" throwing the background out of focus for a pleasing focus on the subject.
With glamour, I am sure they use one or more extra lights behind the subject, illuminating it, killing shadows. So, the distance is less critical. They also do it to over expose the background to eliminate background distraction from the beauty in foreground, so using distance for focus separation is less relevant also.
Finally, the distance should be determined by the goals of the picture. An environmental portrait would want more focused background than a studio shot.
This is all just my opinion, since portrait work is not my strength.