Demystifying the Parallax Actionscript

well guys, I need help

I while ago I found this awesome parallax menu:

http://www.flashkit.com/movies/Effects/Eye_Candy/Parallax-Ashley_W-6442/index.php

the thing is that now, I am trying to modify it or to make my own menu out of it… but it seems impossible. I don’t know which part of the code to modify so it can also work in another workspace size. I mean, the images roll in the right way and everything works perfectly if it’s the same size and the workspace is the same BUT if I change the size of an element of the menu then it will stop rolling before time or will keep on rolling more than necessary. I’ve figured out how to modify speed but nothing else.

Please, help!! This will be useful for others too