Scrolling sub-app

I have been trying for a while to do this.
Basically I have an application with a subapp inside. The subapp contains the scrollbar (as I can't put the scrollbar overlapping the subapp itself).

What I want it to do is when the scrollbar is moved down, then the subapp will also scroll. I am currently using:

+ Always
- Centre Y position of screen to value of subapp

It works, however it doesn't work very well as if you scroll the first few values, the screen doesn't scroll until you reach a certain value (frame height I think).

Any help appreciated