The butt plate to which you refer is inlet first along the butt end as close as you can, and then the upper part - the return - is inlet straight down. But with those progressively smaller transitional sections, the plate CAN move forward as you inlet the top part, thereby taking up any gap you might experience in the butt part. So in practice, the whole affair is inlet from the heel, down and forw'd, toward the trigger guard. A guard that is asymmetrical on the return must be inlet straight down, so you have to have the butt plate fitting almost perfectly before you start with the return. Small gaps near the heel can be taken up with judicious use of a nice hammer. But not at the toe.