Add A Gray fill layer at the top and set to saturation, you can then toggle this layer on and off. this will allow you to judge values , not paint them, this layer just evaluates, then use a layer underneath it, then paint in the values for the back ground, midd ground and foreground, depending on the composition and lighting the background would be lighter and hazy/blurry, again depending how much depth of field there is, the midd ground darker and the foreground even darker. once you have this you can then paint the lighting according to the values.
If you were to turn this in to matte painting at some point and add images, the saturation layer allows you to use curves to adjust that image to the values in either b/g,m/g or f/g areas.
Its a good image you have there but why not play around and add lighting to the trees as is the moon ?.
Good stuff.