You can adjust the brightness from your keyboard, using the f14/f15 keys.
Anyway, you don't need to adjust it, because you are going for the lowest brightness setting.
Reduce the brightness to the minimum, and then start the calibration process.
As far as I know, if you needed more brightness for a different task than photo editing, you can turn the screen up again, and then back down for editing.
But, to be honest, I got used to the lower brightness, and wouldn't want to crank up the display.