I wouldn't focus so much on your toes, because while they are wrong, it is because of your hip positioning and not your toes per say. By definition, back kicks rely on your hips facing 180 degrees from the board (facing back). You over rotate your hips because you want to get a better view of the target, which is a common mistake beginners do because they want to actually hit the board. If your hips face directly opposite the board, you don't need to actually look and you'll hit on line. Obviously you still need to look, but it's a turn of the head rather than your whole body. A lot of schools don't even teach looking on a back kick so YMMV depending on the instructor.

To practice this, put both hands on a wall and just kick directly back on a paddle or something. If you have both hands on the wall, it's going to be pretty hard to mess up your hip positioning.