Chuck is it leaving behind some of the shadow? Or it is taking to much of the shoulder with it? If you use the hard edge brushes then it "should" give you a clean edge. Normally any residue is left because of "anti-alias" if you use the keystroke shortcuts the increase/decrease the brush tip you could zoom in your mask to make a better edge.

The keystrokes to increase or decrease the tip size and edge are the "bracket" keys:
} = increase tip size
{ = decrease tip size
shift + } = increase hard edge
shift + { = decrease hard edge (softer blur)

Let me know how you're making out with it,

:)
retrocity