Advanced camera panning

I have the following code which lets me pan an image with mouse movement in a simultaneous x and y pan. (I will post my code following this post). I have made sure that my background image is a bit larger than my stage to account for movement. My results show the background moving upwards the top stopping nicely at the edge of the stage, however the bottom comes up just a little bit to far revealing bare stage. Why does the left and right work just fine but the up and down are screwed up. The "X" script is identical to the "Y" script. (with the exception of the x and y's).