Help with calculating

If I have a mask that say is 200px tall. The object it is masking(call it listObject) can vary in height 200px to say 600(doesnt really matter).

I have a button that when I scroll over I would like it to move the ListObject along the y axis but only to the bottom of the mask. So the last object in the list is revealed and it stops scrolling so to speak.

How can I calcuate the y position on scroll over.

I know I can start by creating variables for the mask.height and listObject.height, but dont know how to use them to calculate.

Does anyone know how to do this?

Thanks in advance.