How to set up a new cabinet for rear projection tv?

I have a 72 inch rear projection tv I picked up for next to nothing. SCORE!! I am setting up my new room, and I will be building a new entertainment system to house it, along with my game systems. My question its this: I know there is some sort of math for determining the center of the tv to the height of the viewer and to determine distance. I will be alone most of the time when viewing it. Does anyone know the formula, or where to go to find out? Since it will be handmade, I want it optimized for conditions. Thanks!!