How can I change the percent of screen brightness that changes every time I press the shortcut?

From bug 207473 "Screen brightness double level changes on Dell laptops".

• If your problem is an error in hal or acpid you can circumvent it by adding blacklist video in /etc/modprobe.d/blacklist.

• Testing can be done by issuing:

  /etc/init.d/acpid stop
  /etc/init.d/hal stop
  

  If this works 2 programs are changing the brightness level at the same time (thus doubling it) and you should add yourself to the affected people on the bug too (it's old and never got fixed it seems?).

It's not possible, you can confirm it yourself.

Open a terminal and run:

cd /sys/class/backlight/*

To get the current brightness level:

cat brightness

To display the maximum brightness, run:

cat max_brightness

The brightness level can be a value between 0 and the output of max_brightness. To change it to 5, run:

echo 5 | sudo tee brightness

Using these commands, you can confirm that brightness cannot be changes in "percents", just in "levels".