What i did

This one is up and saved as brightness-up.sh : #!/bin/sh

max=`cat /sys/class/backlight/intel_backlight/max_brightness`
cur=`cat /sys/class/backlight/intel_backlight/actual_brightness`

cur=$((cur + max / 4))

if [ $cur -gt $max ]; then
    cur=$max
fi

echo $cur > /sys/class/backlight/intel_backlight/brightness

And this one is down and saved as brightness-down/sh : #!/bin/sh

max=`cat /sys/class/backlight/intel_backlight/max_brightness`
cur=`cat /sys/class/backlight/intel_backlight/actual_brightness`

cur=$((cur - max / 4))

if [ $cur -lt 0 ]; then
    cur=0
fi

echo $cur > /sys/class/backlight/intel_backlight/brightness