Scroll speed formula

Hello again!

Im trying to create a formula for an scroll bar. The problem is, bigger the content to be scrolled, lesser the _y variation of the scroll.

Just see the example:

The “LEFT SCROLL”, has a big content, so the arrows (up, down) increase/decrease the bt_scroll, increasing or decreasing the content _y.

On the “RIGHT SCROLL”, a small content just do the same thing, but should be bigger the jump of the bt_scroll to reach the end of content faster.

I cant figure out how to make this formula for any content height.