Ideally your eyes should be able to look at the centre of the screen without looking up or down.Therefore that is going to be determined by your seating height and whether your seat back is inclined and at what angle.
In my case the bottom of my screen is 3-4 inches higher than my eye level but I sit on a chair that is slightly reclined which leaves my eyes at the centre of the screen without looking up or down.
I watch films about 1.5 foot off the floor which makes me look at the centre of the projection, just as kbfern recommends. I find looking up tiring and uncomfortable. I never understand people who stick their flatscreen telly high above the mantelpiece.