[Zubersoft]

Assuming you are already in the vertical scrolling display mode, I think what you will want is the "Scroll Down ot Turn Page if at Bottom" pedal action. Select that for your pedal under Settings->Touch & Pedal Settings->Pedal Actions and then change the Scroll Amount to 50% at the top of the screen (or whatever amount you want per pedal press). You can also control the scroll speed. Let me know if this gets you the behavior you are looking for.

EDIT:
Looks like Johan beat me to the punch