Very true. In my indoor test I tried to maximize flare as much as I could by filling the frame with a white background, throwing as much light as possible at it and making the black "target" in the center as small as I could without making it difficult to read the density of it in the resulting negatives. As noted of course, using a reflection target in this way obviously limits the maximum difference in luminance between bright and dark. I better way indoors would be to have a brightly backlit target filling the frame, with an opaque dark patch in the middle. This would create more flare. It might better approximate the situations I'm often dealing with in my photography - predominantly dark subjects and/or night shots with lights in-frame.
I have a question about your methodology. How would you fill the frame with your step tablet? This is a problem I struggle with in designing these sorts of tests.