Dkeller, I'm glad you are giving it a try. It's very interesting, the capabilities of this. I am imaging CTB-1 right now. It's an extremely faint object, although I don't think as faint as OU4. I know it's been a challenge for people in the past. So far, with 120s Gain 200 subs, I'm seeing it in each individual sub. I expected going in that I might have to stack first to see anything, but it was right there without any fuss. Now I am particularly excited to give OU4 a try. It's both faint, and OIII, which is kind of a one-two punch combo in narrow band.
Regarding offset. Changing it from 21 to 50 won't result in some huge difference. The main difference is the background....if you set it too low, you will usually see banding from the bias signal in the background. To see that in SharpCap, you gotta expand the Display Controls panel, crank Display Contrast up to around 8, and Display Gamma to maybe 1.3. Then start fiddling with offset (which is the Brightness setting). You should see what I am talking about.