Max brightness. The contrast curve doesn't change with brightness so the only reason you would pick a specific luminance level is to soft proof a print under controlled conditions (i.e. making sure your screen is brighter than the environment but not brighter than the lightbooth you are using to look at the photo).
LCD monitor test images Head over here and check the differences between the black values. See if you can see all shades. If not crank up the brightness till you can.