Miles,
I set them all on 6x, and since it's rare to find a variable that doesn't include 6x with less than 36mm objective. Setting the scopes on 6x results in an exit pupil of at least 6mm, and usually larger. Supposedly the "average" human eye's pupil expands to 7mm in darkness, but I've found 6mm more like it in any sort of hunting light, so a 6mm exit pupil provides all the light our eye can use.
Magnification also affects perceived brightness, essentially due to placing the object closer to the shooter, but I designed the test to be as close as possible to a pure comparison of optical quality.
The black and white chart also eliminates color from the equation, making the test just for brightness and sharpness. Individual perception of color varies, often considerably, and can make a specific scope appear brighter to one person than another.
