When the plant foot comes down, I understand it should be perpendicular to the pivot foot and point toward the plate, but if you were to draw an imaginary line from the pivot foot heel down the mound and parallel to the plant foot, where should the plant foot be in relation to the imaginary line?
I would adjust where you start with your line. When all your weight is on your back leg (pivot leg) and your stride leg is bent at the knee, your center of gravity is on the inside part of the knee of the pivot leg, not towards the heel. This slight adjustment of drawing a straight line from the heel compared to the inside part of the knee of your pivot leg, might cause you to open up too soon.
But, depending on where you are situated on the rubber (I preferred the right side), the best route is the shortest route. Your stride foot should land on that direct line to the plate with your toes slightly pointing to the right if you’re a RHP and vice versa from the LH side.
It should be on that line…you need everything working north and south