What you should do is go to a photo shop and pick up the 'Kodak Grey Scale' card, which should be about $10, and it can be placed in front of the screen, for a close up shot, and then one further back. There is a LARGER one out there, that is about 14" long, not the 9" one. The larger one is only a couple of bucks more, and is much better for these purposes.
The card has, 20 f stops of grey level, from completely white to completely black.
This allows us to calibrate our understanding according to what we know of the grey-scale card, and allows us to compensate for irregularities and inconsistencies with the software encoding, decoding, our display at our end, etc. If the twenty gradations are truncated in some fashion by encoding and other such considerations, it will show to a certain extent when using this card.
I am going to send this card in (two different versions, but both identical -one older, and the other newer) to a place that will calibrate it to the grey % numbers instead of f stops, and to the same level and standards as the National Bureau of Standards. The gentleman involved is doing it to help us out here (nobody knows who it is, or where it is being done, and it has to stay that way,.... shhh......)