In an effort to establish some common ground, Manfred, could we agree on something other than, quote, "close to"? Like for example "roughly" or "between this value and that value"?
The term "close to" is rather subjective, IMO. For myself, numbers up to say 3 are close to zero but it would appear that even around 50 is close enough for your good self.
Somewhat relevant -
Poynton has "Macbeth" black with a Y value of 3.1% and a white of 90%, neither of which I would regard as being "close to" zero or 100% respectively. Others here might or might not - but they seem to be keeping out of this titanic discussion ...
I'm hoping that you are familiar with this table:
https://xritephoto.com/documents/lit...Data-1p_EN.pdf
In particular, please see patches 19 to 24 where it is quite evident that the white patch is not pure white and that the black patch is not jet black.
If I were Tarek, interested only in the difference in output brightness between one converter and another, I would concentrate on the mid-gray patch #22 and would ignore the black and white patches as irrelevant ...