Brightness setting is reset to maximum at login
A possible solution: Add
xbacklight -set 80
to the file
.xinitrc
This will set the backlight to the desired value (80) whenever X is started.
.xinitrc is the configuration file responsible for the X-Server.
Some laptop's have that setting in the bios, like this: 
You normally press the Del or F2 button during startup to get into the bios, the key is written on the first screen you see when your computer starts.
But I'm not sure if you can change these settings from ubuntu.