When you have to adjust the display brightness during the calibration on a Mac, please set the brightness either using the function keys. You can see the level of brightness in small "blocks" on the monitor. The normal F1 / F2 keys switch a whole "block" up / down, which is quite rough. These "blocks" can then be adjusted in 1/4 steps using Shift + Alt + F1 / F2 keys. So you are able to restore the brightness setting afterwards via the number of "blocks" - you just have to remember and write down the values.

Alternatively your can use the brightness slider from the System Preferences -> Displays setting.

On macOS 11 Big Sur and higher, please do not use the brightness slider from the NEW Big Sur Control Center.