Just an observation...
On my last several guns, it seems I've used more gouges and less of the cabinet maker's rasp for the final butt plate fitting. Mostly number 3, 5, and 7 gouges, cutting sideways across the grain.
The end result leaves a swell of wood in the center corresponding to the hollow of the under side of the butt plate, which seems to help seat the butt plate.
It's by no means a exact fit in center of the plate but it seems to help it find its proper position.
Jeff