depends on how you plan to place your image.
i do CIW (constant image width). for a 16:9 image, full screen, i am looking straight at 1/4 from the bottom.
for 2:35/2:40, i lense shift down so the image is still at the bottom of the screen.
i then mask the top.
i then look straight at 1/3 from the bottom of the screen.
this is an extremely satisfying view !
if you do not lense shift. do what BMX said.
now. i have seen a, i don't recall exactly, 190" scope screen. and he had it about 1 1/2' from the floor. and it was awesome !