I ran through the layout using AvFan's 40" eye level. At 13' from seat to screen, you will be looking up at an 11 degree angle. At 14' from the screen, you are looking up at a 10 degree angle. If you use those angles to adjust your support, that puts your line of sight perpendicular to the screen. Of course, as AvFan points out, plasma's are pretty good about off-axis viewing, so the picture quality may not benefit greatly. The bit of downward angle may help with glare from ceiling lights, but that will depend on a lot of other variables as well - it may not help at all, and of course if you are watching with the lights off it'll cease to matter.