Help - Interactive Image Panning in AS3

Hiya, I am trying to implement the interactive image panning script tutorial from this site into my AS3 movie. The problem is that the image I want to pan is contained in a masked movieclip which has a width smaller than the stage.

I have no problem translating the script in the tutorial into AS3, however I’m having trouble setting the bounds of movement so that it’s constrained to the mask’s position and width.

Does anyone care to take a look at my simple flash file and help me with making this work?