scroll-indicator-2.jsx

const scrollElementHeightPercent = 45;

export default Example = () => {
  ...
  const [scrollViewHeight, setScrollViewHeight] = React.useState(0);
  
  const scrollPerc = (contentOffset.y / (contentSize - scrollViewHeight))
    * (100 - scrollElementHeightPercent);
  
  return(
    <ScrollView
      ...
      onLayout={e => {
        setScrollViewHeight(e.nativeEvent.layout.height);
      }}
    >...
    </ScrollView>
  );
}