Your reasoning is reasonable, and in theory you could just use one single reference object. But in practice DSOs are different from each other and have different brightness profiles, so I'd build a library of sample objects as you suggest.
(what I usually do instead is try with my "mmmh this should be fine" settings and see what I need to correct for that specific field I'm capturing at the moment)

And also, as usually we try to make pretty pictures, you have to consider field stars to avoid pixels blooming: stars are brighter and smaller than diffuse objects (so, for example, I ramp up exposure time until stars get worse). See en.wikipedia.org/wiki/Surface_brightness#General_description

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