Focus height (as I will call it) depends on scroll speed. And scroll direction. I'll be assuming downscroll for this post.
The slower your scroll speed, the lower you can look without causing problems by giving yourself too little time to read the notes.
The faster your scroll speed, the less time you have between when the note enters the top of the screen and when it hits the receptor at the bottom, which means you should probably be looking a bit higher up on the screen at higher scroll speeds.
the exact location you look at is personal preference, but generally you want to be slightly above the middle (if you want to know what looking at the middle is like, use flashlight, it's probably lower than where you naturally want to look anyway).