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As Ray says, I only use 2 gain values (rarely, a 3rd) and have built dark libraries up over time. I always use offset 50 also.
Yeah the background ADU is the K value in PI, if you hover the cursor over background sky and not nebulosity or any other light emitting object. It might help to change the probe size to 9 or more pixels (click on the "play" button at the bottom where you see the K values). I often use a gain of 139 (and offset 50) and aim for a background ADU of around 1200 or better (looking at uncalibrated light subs).
Your (very) low offset will skew your figures relative to mine, but at gain 139 offset 50, a short dark frame of mine will result in a mean/median of around 800 ADU. I'd imagine that yours would come out at ~120ADU?
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