G'day Geoff. I see you had a busy weekend
From what I understand, you want the spike in the histogram to be 1/3 to 1/2 way from the dark end (left end) of the histogram. Can you post examples of the ones you're getting?
I have my "lightbox" solution coming in the mail at the moment, so I haven't actually attempted this myself yet. But what I was going to give a go is to set my DSLR to AV mode, keeping same ISO, and just click and the camera should choose the right speed for the exposure. Chimp the histogram, if it's ok, good. If not, maybe play with exposure compensation until it is.