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Ok. Sounds like you're most of the way there. I'll discribe what happens with my set up although it's likely you have darker skies than I have and you're using an 8" SCT where as mine is a 5".
Yes continuiosly taking exposures to reduce the size of the donut is a crude way to get focus, it's exactly what I do or at least how I learnt my setup. I haven't been imagining long ( 9 months or so ) but it didn't take long to learn roughly how far to turn my focuser and in which direction from my 21mm eye piece to prime focus in the camera. This only gets you close though. I then fit my bahtinov mask ( home made for about $5 ). I then take a shot while pointing at a bright star. I keep doing this until the defraction lines are right then the focus part is done.
One key thing to keep in mind though is that your goto might not be 100% accurate. What I normally do is goto the object with my eye piece in and fine tune it to be spot on. I then save the object as a user object. I then slew away and goto my user object to make sure it is framed exactly where I want it to be. Only then do I fit the camera and go through the focusing process. i generally use the brightest star near the object I'm planning on imaging.
Your darker skies and extra apature give you the advantage of being able to see the nebula you want in an eyepiece , I can't. I have to look for star patterns to know I'm where I need to be.
When I start taking my exposures, unless it's super bright like Orions nebula, I actually can see very little in my review. Even when I load them on my computer, in each single sub, I don't see much apart from stars. It's only when is stack them I can actually see what I've taken.
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