that is amazing when you consider how short your subs are.
May I suggest that you keep adding to this image. Capture 80 or 90 x 10secs. Fill your card!!!
I think you might do best to turn on your ICNR instead of taking separate darks. When I was at your stage, any sub I took that was under 1 minute, I found it more convenient and the images looked better. It also saves heaps of time when you are stacking.