I faced the same problem on my Dell XPS. Currently there is no other way to save it according to brightness key.
Find your brightness control path, execute
the output will be something like
Copy that path and try executing
sudo su -c 'echo 5 > THE_PATH_FROM_FIRST_COMMAND'
it will be like ->
sudo su -c 'echo 5 > /sys/class/backlight/acpi_video0/brightness'
If it succed,try with different value for
5 and choose your preference level.
/etc/rc.local file to execute this command at boot up.
gksu gedit /etc/rc.local
exit in the file, add the following line,
echo 5 > THE_PATH_FROM_FIRST_COMMAND
You can change value
5 to anything.
How do I prevent Ubuntu from changing my laptop screen brightness level?