When adding slice markers, users will generally be zooming in and out a lot. By moving the ‘Zoom’ button to the left side (ideally position 3), they can keep their left thumb on the four primary screen buttons (slice selector, slice position, zoom, add slice) and their right thumb on the +/- buttons. This will make the process quicker by not having to shift hand positions as much.
Adding slice markers is tedious/annoying when having to shift between the left and right thumbs accessing screen buttons constantly.
Quicker editing of slice markers.
Use the existing screen button layout.