Value checking and mirroring

I often flip the canvas to check composition. A good composition should work mirrored (and even upside down). During this stage, I check the black and white values by adding a layer on top of the layer stack and setting its properties to Saturation. Then I fill this layer with solid black. Now I simply turn on/off the layer to check black and white values. The common value balance is darker for foreground, mid for mid-ground and lighter for background (sometimes referred to as atmospheric perspective).