Sometimes it is only possible to increase and decrease it by a sections like from 1 to 3 any nothing in between can be selected, otherwise you should get software that you can get a slider so you can manually slide the brightness according to your preferences.
Better explained, using the sound as an example. By using the shortcut on you keyboard, the sound can also just increase and decrease by amounts that is default by the key, when you use the slider of the window of the computer, then you can manually change it to 0.5 or 0.8 just as you would like it.
Hope you understand, so look for something that can bring up a slider for the brightness - there should be software on the internet....