Dynamic Scrollbar (scrollbutton) Height

I am currently trying to adapt a scrollbar to change the size of the scroll button depending on the size of the area its trying to scroll. The scroll bar is a full screen scrollbar which scrolls a movie clip.

Ive tried a variety of different ways to achieve this but have gotten myself stuck.
ActionScript Code:
[FONT=Courier New][LEFT]resizer.[COLOR=#0000FF]onResize[/COLOR] = [COLOR=#000000]function[/COLOR]COLOR=#000000[/COLOR] [COLOR=#000000]{[/COLOR]
[COLOR=#000000]var[/COLOR] contentHeight = contentMain.[COLOR=#0000FF]_height[/COLOR];
[COLOR=#000000]var[/COLOR] stageHeight = [COLOR=#0000FF]Stage[/COLOR].[COLOR=#0000FF]height[/COLOR];
scrollbar.[COLOR=#000080]scrollbutton[/COLOR].[COLOR=#0000FF]_height[/COLOR] = [COLOR=#0000FF]Math[/COLOR].[COLOR=#0000FF]round[/COLOR]COLOR=#000000[/COLOR];
[COLOR=#000000]}[/COLOR];
[/LEFT]
[/FONT]

So far its changing the size but not correctly… Does anyone know of a tutorial around somewhere that explains this type of scrollbar or know where I’m going wrong here?

David