Walter, could you not perhaps use a lightmeter or a color analyzer (if you have one, set to white) to read the variation on the baseboard illumination, iteratively adjusting and moving the meter abt until you've got it fairly even? I admit to not having tried the meter, not sure if its sensitive enough. I have a simple photocell densitometre I made 30 years ago (&#33 that I use for setting new f-stops or time when I change magnification, which will also do that function.