31" seems quite high to me. But then I prefer looking slightly down at the display, mine is quite low, about 18" off the floor. Technically you eye should fall at the center of the screen when in your normal viewing spot, I surmise that is where you have it now.
People watch TV differently. I sometimes sit, sometimes lay and sometimes I stretch out on the floor. No way to hit all those spots, so my suggestion is find what is comfortable you and go with it.