Fitting help

I am having difficulty getting the classic shirt to fit me properly. I started out by taking my measurements (H: 40, W: 31, B:37, UB: 35) Since the different between UB and B is only 2" I started with the size 10A/B (although I had my suspicions this wouldn't work well since I wear a 32DDD bra and have lots of trouble finding button up tops that fit properly) The 10A/B was WAY too big so then I went down to the size 8 - still way too big - so I switched gears and decided to use the C cup and just pick a size off my UBM, I made a muslin using the size 4C. It is better but still feels loose over all and now has a pucker from the shoulder to the bust area. (see attached pictures) Do I go down to the size 2C, or maybe 2D? or 4D? or just make a custom size by playing with the dart on this muslin? Thanks! Karin.