Hello again.
While working with overlays one option seems to be missing - the ability to see and set their position as numbers. It's rather hard to achieve exact same positioning with the zoomed-out dragging interface. There seems to be enough room for some number fields, it could be done like this:
Perhaps also with a "Center" button next to position which resets the overlay, and a "Scale to fill" button next to scale.
As for values percentages seem to make sense as they are reasonably resolution independent. Positive and negative values would need to work for position and only positive for scale.