The Brightness setting is basically the black level and the Contrast setting basically the white level. You want the Brightness setting to be at the point that things marked for black cause the Ruby to put out as little light as it can, but things marked just for just above black get a little more light (or very near to that Brightness setting). Lowering the Brightness below this will crush those things that are supposed to be above black and raising it too much will hurt the absolute black level and on/off CR. Leaving the Brightness where it should be and lowering Contrast a lot hurts the contrast ratio. An ND filter basically allows you to dim the whole image while maintaining the correct Brightness and Contrast settings for good CR between white and black and good shadow detail.