ScrollablePositionedList initial scroll offset

I’m working with ScrollablePositionedList in flutter for a while. From the document I found, it said ScrollablePositionedList works slightly the same with ListView. My problem is that I want to set the initial index of the ScrollablePositionedList in the position around 70% of screen height from the top of the screen, and I intend to do with by setting the pixel for that initial index. However, I’m struggling to find how to set initial scroll offset for the ScrollablePositionedList with ItemScrollController. Please help me