Lazily, I just eyeballed it without changing the brightness. But I guess you can temporary change the brightness to match your surroundings. And then after setting it put it back to auto.
The whole point of this night mode is to limit the amount of blue light reaching your retina, for the few hours before you sleep. If you can stand it, you can set it all the way to the right, to orange. Most people won't tolerate that, so setting it to your (hopefully yellowish) house lighting should be tolerable to everyone. As it is comparable to reading a piece of paper under the same lighting, your brain will easily adjust to it in no time. Keep an open mind for a few days and stick with it.
