With my PA241 I have the possibility to set the Black level with the Multiprofiler. I just don't know how to find out a good black level. Visual assessment seams difficult to me.
there are three main reasons to boost the black point ... if the monitor is used to edit print related images (for video editing it's completely different):
1.) all the measurment devices have accuracy issues at very dark onal values. the higher the black point the better (i.e. more accurate) the calibration curve
2.) too high contrast is tiring for the eyes. Too, you won't see "more" at higher contrast (rather the contrary)
3.) monitors have a much higher contrast than prints. boosting the black level will bring the monitor closer to a contrast similar to that of a print (photo print).
I am currently working with a contrast at around 1:340 and that's quite good here for me.
So... when the brightness of your monitor is somwhere around 120cd/qm you should try 0.3cd/qm for the black level. Contrast ratio will be around 400:1 then. If your monitor is closer to 160cd/qm you should try 0.4 for the black level to end up with the same contrast ratio.
Don't worry f the contrast is lower than 1:400. But it should definitely not be less than 1:256.