This is an answer from a similar question I replyed to a while back. A few simple measurements and you'll be able to figure the exact riser height you need:
"Measure the height of a person seated in the front row (floor to top of head (as well as eye height, to be used later)) and subtract the height to the bottom of the screen. Divide this by the distance to the front row viewers eyes (in inches), and you have the RISE/INCH measurement. Multiply this by the distance to the rear viewers eyes (in inches). Add the screen height, and you will have the height the rear viewers eyes need to be, to clear the front row heads. Subtract the height of the seated viewer's eyes, and you have the riser height.
Here's an example using your given measurements and some guesses:
Height of 1st row heads: 42" (guess)
Height of first row eyes: 36" (guess)
Height of screen bottom: 24" (given)
Distance 1st row viewing: 132" (guess based on given)
Distance 2nd row viewing: 192" (guess based on given)
42-24=18
18/132=.1364 RISE/INCH
.1364*192=26.2
26.2+24=50.2
50.2-36=14.2" Riser
Using the above numbers, in order for the rear viewers to see all of the screen above the front viewer's heads, you would need a riser of no less than 14.2". In order for an 8" riser to work, you would have to raise your screen up a little more than 12".
When I built my riser, I added a couple of inches just in case there were taller people up front and shorter people in the back; or in case the back row felt like slouching down a bit, and the front row didn't. So far, not a single view blocked!"
Rob