Rule of thumb is to run the contrast at max, brightness so that you can see differences in the darker greys. Ideally, on a PC you should be able to see all 17 of the greys on the one I'm going to attach, with about equal differences between. Macs are going to have trouble with the dark end, incidently, because they run different Gamma factors. Going past that, programs like Photoshop include ways to calibrate your monitor to a standard, however it is very, very subjective. I used to cringe when I taught that part in my Photoshop course, walking around the room I could see about half the people were ajusting all wrong, sort of using the settings from Outer Space. Some just don't have a clue!