The patterns for setting brightness are a bit easier to use if you can display below black. You don't list what source you are using to play the video, but most HD players should pass below black to the TV by default, except for computers or the PS3. The PS3 can be set to send the entire video to the TV, but by default it doesn't. Another possibility might be that the TV, or possibly a receiver, could be cutting off the signal below 19.
Even if your display is showing 17 and 18, it can be rather difficult to tell nearby levels apart. Generally I use a very dark room with the black clipping pattern, which makes near blacks a bit easier to pick out. Commercial disks don't show any levels lower than 19, so it's reasonable to just make sure that the brightness setting doesn't cause 19 to stop flashing.
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