After a few more changes I think I’m ready to use my muslin as a pattern. Thank you so much for your comments. A special thank you to Kenneth D. King, I was very grateful for your comment, I’m a great admirer of your work and books and was surprised you saw my post on this and took the time to comment.
Below the first change: added a thicker shoulder pad, it definitely helped (change to the left side)
Then I made a slight change to the armhole and deepened it a bit. As Gail suggested I also did a little swayback adjustment. Time to transfer the muslin to a paper pattern I think. What you see at the upper back is not a round wrinkle but the camisole I’m wearing underneath.
Since making this muslin I’ve looked at a lot of photos of garments I made in recent years and I see the problem more often than I like. I’ve learned a lot from this muslin, blouse patterns have to be altered too. Also that I have to refer to my fitting books again that have been accumulating dust and keep looking with a critical eye.
It’s definitely worth all the time spent. A jacket like this I will probably wear for quite a while. I still wear the jacket below (same problem of the upper back being too wide at the armhole). It’s from May 2010. I intend to construct the above muslin jacket in the same way, which hopefully will lead to another jacket that will last for a while (can you tell that I’m not dressed following the latest fashion?)