Back Rub Etiquette

I enjoy back rubs, but sometimes when a co-worker is massaging my back, I start to wonder--where are the boundaries on a woman's back?  How far around the side and down the front is permissible?  Lower than the collar bone?  Farther around than the mid-axillary line?  Or the phlebostatic axis?

To clarify this puzzling issue, I came up with a chart to illustrate the DANGER ZONES  not to be breached in back rubs:

DANGER!!

Side View (Left)

DANGER!!

Rear View

DANGER!!

Front View

I asked my husband what he thought the perimeters should be when a guy rubs my back.

 

He touched the very center of my back and said, "Right there.  That's the limit."

I think he's anti-research.