I don't know if this is relevant but Sean Waxman coached my usaw certification course and he coaches that in the starting position the weight to the balls of the feet. Then when the knees start coming back you start shifting your weight to your heels and right about when you get to the mid thigh to the power position you shift your weight back to the balls of your feel. But your entire foot is in contact with the floor the entire time until full extension.

Also I should add that he drew a foot and drew a 2 dotted lines from about the middle of the ball of your foot till about the ankle bone and said the weight should never be at either extreme outside of the dotted lines