try slightly lifting the toes off of the ground as this will shift all the weight through the heels. Make sure your stance is at least slightly wider than shoulder width, if not, wider. Doing so will enable you to squat with your glutes and hamstrings, which is the powerlifting style.
If that does not help with keeping the shins perpendicular to the floor, then try stretching your calves, as tight calves have a tenancy to keep people from squatting through the heels.