First thanks for the feedback!
1) I made a really simple example so people can understand how to tune/use the scrollbar... the position calculation of the box dont have checks so its probably a problem in the formula. For the real values of the scrollbar see the caption. The values seems to be ok.
2) Not sure to understand this... but perhaps has something to do with me using the same max values for vertical than horizontal instead of using a bigger horizontal value.

I'm doing right now another control (almost finished) that use the scrollbars and have a way better and complex example.