The brightness (luminance) value is something that changes depending on what viewing environment you are in, as well as what display you have. For LCDs, the typical setting is supposed to be between 110 candelas in a very dark or dim room, while the maximum setting to be around 130 candelas in a reasonable lit room. When doing color work at all, keep all light dim and not directly behind you to rid of glare on your display. I have always set my display at around 128 because that is the "hot spot" in which my display shows its maximum color gamut while being at my desired brightness level. Where I work, we have our 20 inch ACD set at 120 because they seem to function best around this brightness level. Things may be different for a 23 incher, but I would recommend starting at 120 and go down from there if things still seem too bright. If you have any more questions, let me know.
Mac Pro 2.66 GHz, the works. Mac OS X (10.4.10)