What Don says works very well, and his neck fatigue issue is very valid. I find looking down to the screen preferable to looking up, so the center of my screen is roughly in line with my eyes.
When watching a 2.35:1 movie, I align it with the bottom of the 16:9 screen and find that more comfortable (strange I know).
At the cinema, I prefer to sit at the center to back rather than at the lower front for the same reasons.
It's all personal preference really, so if you build it for the people who will be watching it the most (you), then you won't go far wrong.
Gary.