Is there anyway to change by what percent the display backlight is adjusted each time I hit the brightness keys? I assume there's a script somewhere I'd be able to edit.

I'm afraid in this case the actual brightness step is hardcoded into KDE Power Management.
It can be found at kde-workspace/powerdevil/daemon/backends/upower/powerdevilupowerbackend.cpp - on lines 202 and 204.

Please note that the actual step is also adjusted by the level of support in the hardware (later hardware specific code will scale the "10" to the level appropriate to that hardware).