You can't adjust the top and bottom of the screen independently. Indeed, the screen is very uniform. You see a difference because you don't look at the pixels with the same angle. This difference decreases with distance.
The best you can do is find a setting where the top is not too dark and the bottom is not too bright.
To calibrate your screen, go to Preferences > Displays > Color > Calibrate.
Choose expert mode and follow the steps. You must be in your normal viewing position during calibration. You can also make a calibration profile with your screen tilted forward and use it for your plane trips.
If you don't like the result, try to calibrate with Supercal wich is a bit more precise :
http://www.bergdesign.com/supercal/
You can also adjust your screen on the fly with gamma control :
http://www.michelf.com/projects/gamma-control/