I did mine dead center/maybe a 1/4 inch high. For reference, I have a Regal pedal mounted on 1/2 thick board. Everything lines up perfect. I did wait until the frames were installed to drill my holes though so I could pull a rope in the area and see how best to line up the holes vs drilling them all in the same location with regards to the floor. Hope that makes sense.
Sounds like predrilling is not really critical.
It’s not. But it is important that you do it before installing the bottom.
If you need help getting a system I can get you one.