Here is the spec for ITU BT.2111, the specification for HDR color bars.

https://www.itu.int/rec/R-REC-BT.2111/en

Having worked in transmission for many years, I can tell you that standardized color bars are still quite useful.

The upper strip of the color-bar signal is 100%; the remainder is 75%.

Below are some sample 10-bit values taken from TABLE 2 of the spec (R-G-B):

100% White: 940-940-940

75% White: 721-721-721

100% Yellow: 940-940-64

75% Yellow: 721-721-64

0% Black: 64-64-64

-2% Black: 48-48-48

+2% Black: 80-80-80

+4% Black: 99-99-99

For setting monitor brightness make the +2% and +4% bars "just visible".