are you going to have a step to get onto the stage? A typical riser height for a stair is 7-8". If you add 2 layers of 3/4" on top you are going to end up w/ 13.5" riser height which is nearly double the standard. If you have a step it shouldn't matter.
One of the things to consider is the vertical viewing angle. Whether you put your seating on some elevation or not will determine where your screen should ideally be. The recommendation for the "best" seats in the house is to have the eye balls in line with the bottom 1/3 of the screen. Factor in screen border and any space (asthetics, lighting, center channel, subwoofer height etc.) you want to have around and bottom of the screen. This will let you know what the height of your stage should be.
BTW - I am at the same part of my HT build right now and even though I wanted a higher stage, I decided to go somewhat lower since my center channel and subwoofer will need (don't have space off to the sides) to be under it.
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