That's an average value of 400ADU (16 bit), calibrated (dark subtracted. Without knowing the camera or gain this is approximate, but I'd guess 60 seconds would be better. In Bortle 7, with a C8 RASA at F2, ASI183 camera, I do 120 seconds narrowband at gain 100.
I could tell better with the camera info. EDIT: I missed the camera in the title, still need the gain.
Or you could.
Look at the average value of that dark subtracted light in Image-Statistics. Using the camera data, convert that into electrons.
Take the camera data, find the read noise in electrons. Square the read noise. Compare it to the average above. You'd like the average to be somewhere in between 5 and 10 times the read noise squared. That balances out too much read noise (too many short subs) and too little dynamic range (too few long ones).
Lowering gain to lengthen exposure can be a good idea. It depends on the read noise of your camera.
Broadband is hard in your skies, even with HyperStar (which helps).. It will work significantly better with a mono camera and LRGB filters and processing. With HyperStar you'd need a filter slide, and some effort.
Part of your image (the bright stars) will always exceed the top ADU for you camera. At that point, the stars get large, and, with one shot color, you lose all the color information.
Edited by bobzeq25, 17 February 2020 - 04:28 PM.