I have altered the muslin I made for Butterick 4188 to try and eliminate the pull at the back shoulders. I have made an adjustment for sloping shoulders by dropping the shoulder seam 1 cm at the shoulder end. I also dropped the underarm seam by 1 cm. I think it has improved it. Maybe I could take more out but I am only new to this alteration thing so I think I will err on the side of caution. Also the fabric I am making the top out of is softer cotton than the calico/homespun I have made the muslin with, so I don't think it should be so noticeable.