Erik Heyninck provides this Zone System Grayscale Chart. “You can easily adjust your monitor's midrange brightness with it, as all the grays must be clearly discernible (under the proper lightning conditions).“
“The Zone System is a method of dividing the gray values between and including black and white into eleven levels. Black is Zone 0, White is Zone 10. Zone 1 is dark gray without detail, Zone 9 is light gray without detail. Zone 5 is medium Gray. It was invented by the great photographer Ansell Adams and is very popular with photographers. Nowadays with computers it isn't that difficult anymore, but I remember Sundays in my small darkroom trying to "develop for the lights" in the images I had "exposed for the shadows".
Beneath the Zones I have added the corresponding RGB values. When the numbers in any RGB value are identical, you get a gray. So when you enter these numbers on the chart in the R, G and the B value, you get these grays.