If you're on the 4 yard line, you move it back to just outside the goal line. In regard your midfield example, that goes back to the point I made above - it's as if you're worried about being too mean. A 15 yard loss on a sack at midfield isn't nearly as detrimental as a 15 yard loss on a sack at the 16. So based on your logic, on the sack, the sacked -team should still get the ball at the 8.