Movie clip in margins?

What is the code that a clip (say ‘ball’) is following the _xmouse only 50px from the beginning and the end of the flash animation.

ex: if the animation is 500x200 I want the ball to follow the _xmouse only from 50-450.

something like this - http://www.kirupa.com/developer/mx/followease.htm
but only horizontal and with spacing from the ends.

Thanks