Dave,
By chance, are you using the Night Mode. If not, try Mode -> Day/Night Mode -> Night Mode. This takes it pretty dark, about twice as dark as just dimming the hardware. Additionally, it inverts the colors on Info, Flight Plan Page and Plates, etc to be much easier on the eyes.
I'm not aware of a way to dim beyond the screen hardware drive will allow. That is why we darken it further digitally. Hope this helps.