But completely ignore Fitt's Law. Usually I can speed my mouse to the right of the screen, click, drag. Here I speed to the right of the screen, maybe try clicking and dragging, fail, move in 30 pixels, overshoot, move back and finally can scroll.*
The problem with allowing the customisation of scrollbars is that most people are going to make them worse.
* Personally I use the scroll wheel, but I know a lot of people who don't.