The best answer to this question is to sit in the viewing area. Looking straight ahead imagine your TV. Once you have a point on the wall use that as your center reference. For me my ideal mounting would have put the bottom of my 50 inch at 36 inches from the floor. However i need a bit of clearance for stereo equipment so i raise my mount 4 inches to 40. This is still very comfortable to view from my seating. One thing to avoid is mounting too high. This will cause you to constantly be looking up and tens to strain your neck even with a tilt mount.
Bottom line 30 to 36 is the normal range for the viewing area you can go up to 6 inches plus or minus from that but anything more will cause more neck strain.