How to get HSB values of layer or document in Photoshop CC 2017 (in my case Brightness of layer)

I tried do find a code and one have found:

Get values of adjustment layer

but it doesn't work in CC 2017.

I know that getting Brightness of layer manually is dependent on pixel being hovered but the idea is to get highest or mean value of brightness.

Getting brightness of all values or maximum or mean of a document would be a solution because I could hide other layers at start and get values as it would be only one layer. But how?

Thanks in advance