I struggled with this question when I migrated from an Atik Horizon camera to ZWO ASI294. Initially, I understated Gain and over exposed, partly because Atik graduates its gain measurements in units of 1 dB and ZWO in units of 0.1dB so "20" with Atik is "200" with ZWO.
However, there is a wonderful tool in Sharpcap called Smart Histogram.
I simply line up its dual green bars, and voila! Gain is accurately set! Then slide exposures until the left edge of the Smart Histogram curve is just inside the left edge of these green bars. Exposure is set. This process delivers the desired ratio with 90% accuracy. Tweaking it from there is simple.
You can try this in daylight too using a distant chimney. Exposures there will be much shorter than at night, but you will get a feel for this valuable tool that people rarely seem to talk about. But it works great for me.
Edited by Noah4x4, 30 June 2020 - 04:59 PM.