/etc/acpi/events/ and you should find some files named something like
-up, along with several other events.
Don't edit those, but take a look at the shell scripts in
/etc/acpi/events/asus-keyboard-backlight.sh looks like this: (The comments are part of the script, NOT mine)
# this directory is a symlink on my machine:
test -d $KEYS_DIR || exit 0
if [ "$1" = down ]; then
if [ "$VAL" -lt $MIN ]; then
elif [ "$VAL" -gt $MAX ]; then
echo $VAL > $KEYS_DIR/brightness
Obviously, given that script, the minimum brightness is already 0. It's also pretty obvious how to change the increments.
Perhaps your script is different, or perhaps you will be called upon to write a new script entirely.
I don't know what will happen if you mess with the max_brightness setting, and make it higher than the actual maximum. Probably something bad.
For more information on ACPI events like that, see here: https://help.ubuntu.com/community/LaptopSpecialKeys