I guess I cannot dissagree with anyone wanting a metric to decide what "sky" background is "appropriate". However, consider that this is an art and eventually one should find and do what they wish. I remember when I started out doing this and posting here, I would get some comments that my sky background was too dark. I took that to lesson I suppose. I present no image that contains pixels valued at zero. But back when I started, I saw much too many images with clearly milky backgrounds. Much too high in my opinion. I think more and more, the artists here are presenting deep space galactic images with darker backgrounds and that to me shows a better image. Still some stick to their guns and present milky backgrounds. For what ethic, who knows?
I personally take an attitude that I am not photographing object millions of lightyears away as sky images. As art, I am trying to present these objects as objects in deep space. Unless I am unaware of a human's vision is capable of seeing the Cosmic Microwave Background, space above our atmosphere is essentially perfectly black, except where out visual line intersects a light emitting object. Our suffering through photographing through Earth's atmosphere need not be applied to my photos! If metrics were important or follow some photographer's honor system, would I need to present my Bortle 4 skies with a higher "sky" background for my image of the Virgo Cluster at some higher level as for my M31 taken in Bortle 1? Nonsense! So then what is the artistic reason for setting an objective metric? That said, I do think that a "Space" background of not too dark a background dimishes what can be a harsh appearance and tough on the eyes. But that certainly depends on the amount of stretch and how bright/dynamic an image is or how dynamic an image the artist wishes to present. But if one is practicing your art for purely personal reasons, then make it what you want. If practicing your art for the broader public and commercial success, you will need to learn your customers' wants.
In my personal images that I present as A ExtraGalactic Traveler's View of whatever galaxy I am imaging, I go one step further. Not only do I try not to mimic anything like a sky glow in my images, I actually remove any Milky Way stars, planetary nebulae, other features, as well. Reasoning for the purpose of the presentations are that Milky Way stars are only relevant to how a distant galaxy view appears to those stuck firmly on Terra Firma. Secondarily, removing Milky Way stars allows the viewer a better understanding of the structure of the target galaxy(s), the myriad of distant galaxies, galaxy clusters, quasars, and even stars for galaxies such as M31, M33, etc, which are close enough to see stars within those galaxies. See example
here. My backgrounds for these images I want dark, but not lose background objects, because I want to be sure that the very faintest of the distant galaxies can be clearly seen, even if the viewer needs to zoom in to a larger degree. Another
example. Having a noticable background gives the feeling to the observer that they are not missing anything. (Fact is, since how deeply can be seen is limited by the conditions and optics.)
