ActionScript 2.0 :: Scrollbar And Dynamic TextBox - Data Entry

I'm creating a custom scroll bar in flash, I'm following a tutorial for how to code it in I believe AS2. However there are some differences between the outcome I want and the tutorial I'm viewing. What I wished to create was a dynamic text field which relates to a text file outside of flash. The problem I'm having is that in order for my scroll bar to work it needs a set distance to work the percentages out that make it work e.g. if I move the bar down 50% the content would of moved up 50%. Because my field is dynamic it can be any size depending on the content. Any code that can get or measure the Y size, then use that calculated size as a variable to control the percentages of the scroller.

So far I have a few variables that have allowed me to create my scrollbar and restrict it's movement. The problem is that my scrollpane isn't moving, the script I have to get the scroll pane's highest Y co-ordinate is:
var scrollpaneUpper:Number = getProperty(Scrollpane,_height)/-1

So that whatever the height of Y is when the dynamic text is entered the variable will get it and divide by -1 so it's a negative figure that will lift the text field not lower it. The problem is it's not moving with my scrollbar or at all and I'm sure the rest of the code is right. I've got everything moving but the scrollpane isn't picking up the updated data from the dynamic text box. Instead it is taking the original value from the height of the text box which was 25 and is moving it that far, which means lots get's cut off. Is there a way I can update the data entry so it's not 25, it's as large as the text box is autoSized to?