This might be an engineering or mathematics question, nevertheless giving this a shot here first. There seems to be a clear way, or at least in regards to distance a few, quantifiable explain where to ideally placing a flat mounted tv on the wall. However the title is the question, how much higher may the tv be placed to sit just as even, so the tv lines up similarly to the couch with on varying degree of tilt?
The centerline of the television will change by the sine of the angle of change times the distance from the television.
In simpler terms, for every 12 inches from the television, a one degree change of angle will drop (or raise) the line by 0.209 inches. Realistically, say a 12 foot viewing distance and a ten degree change of the screen, you get a 24 inch drop of the angle of the line.
Most televisions will have a reasonably wide viewing angle both horizontally and vertically, and allow for leeway in the angling of the screen.