Let me give you my assumptions so if someone wants to check my results they can. When I previously calculated the tilt I assumed you'd be looking at the bottom of your plasma. Seemed kind of odd so for this latest go-round I've assumed the line of sight is to the center of a 50" plasma with a height of about 28" and you want to view the plasma straight on at 90 degrees. I've also assumed that your eyes will be 40" off the floor or 15" below the bottom of the screen.

Doing the trig you'd need your eyes just under 6' from the center of the plasma to have the line of sight perpendicular to the plasma tilted at 15 degrees. If you move your seating position back to 13', and again looking at the center of the plasma, you need to tilt the plasma to just about 10 degrees. This tilt amount is more than before because now the line of sight is to the center of the plasma where before it was to the bottom.

Plasmas can be viewed over a 160 degree horizontal arc, but I don't know if that applies to vertical too. What this means is that it may not make much difference in you viewing experience if your line of sight is not at precisely a 90 degree angle.