Tim,
**Disclaimer the advise you are about to receive is from a new guy to HT and may not know what he is talking about**
From my limited experience I have gotten a lot less fatigue from watching TV when I don't have to my neck up or down much from vertical. Obviously the closer you sit to the TV the more this comes into play. Ever watch that Purple Dinosaur with your kids on the floor? After an hour your ready to shoot that dinosaur if it wern't for that ole krick in the neck.
I would sit in your normal seating position and measure the hight of your eyes and then build your stand to a hight that puts the center of your screen pretty close to that hight.
It is actually more comfortable FOR ME to look slightly down or straight on than it is to look up. I would find a sturdy platform that you could use to fine tune what hight is comfortable for your viewing experience and then build your stand that accomplishes that.
Just my .02
Happy Building
