I cannot answer with complete certainty but I have read that 1/3 of the screen should be under your viewing height. So if when you sit your eye level is 36" from the ground, the following would be true.
48" x .333 = 16" (48 is the height of your screen)
36 - 16 = 20"
So to have 1/3 of your screen be beneath your viewing height the bottom of the screen should be 20" from the floor. If I am wrong I am sure someone will correct me.