Take the difference between the bottom of the screen, and the tops of the first row occupants heads, and divide it by the distance from the screen to the center of the front seat occupants heads. This will give you the rise per inch measurement. Then measure the distance from the screen to the rear row eye position. Multiply this by the rise per inch, and you have how high the rear row eye level must be to view the screen. Subtract from that, the front row eye height, and you have your riser height. This is assuming your front row seats are the same as the rear row seats. You might also want to add a couple of inches, as this formula gives the bare minimum clearance.