Chris, Shiraz did alot of work on this and produced charts for things like optimal exposure time for various f #. His work was all based on the ASI1600MM-C.
I have played around with different gain setting on my 1600, but always come back to Unity. If i am very short of time i will go high gain to complete say some narrowband i am gathering to infill a RGB image.
I never use subs of more than 5 minutes and that is only for narrowband. For broadband your optimal sub length us determined by your sky glow. ADU targets can pin it down, and i assume you can monitor that with SGP or some other tool.
Have a search for Ray's charts. Ok did it for you.
Look here:
http://www.iceinspace.com.au/forum/s...0+exposure+ADU