Laptop display brightness adjustment keys

[Matt Connell]

Software: XFCE ver 4.18 
Hardware: Lenovo T420i


How can I change by how much the brightness adjustment keys change the
percentage?  The keys work, I can brighten and dim the display as
desired, all the way from 0% to 100%, but the value by which it
increments is... strange.  Someimtes each adjustment will change the
brightness by 6%, sometimes 7%, without any rhyme or reason I can
discern.

Contrast this behavior to the volume keys that increment by a nice even
5% every time.

How can I adjust this value for the brightness keys?