Easing Scroller to a fixed MC height

Ok. I put this scroller together, I’m sure my methods are a bit off, but it works. The problem is, everytime I need to add some more text I have to go in and guess what to put on the down button as far as my _y positioning.

What I’m trying to get an answer to is:

What is the method of getting the scroll arrows to know the height of a MC.

Attached is my scroller.

Thanks,
-t