One of the things I see most frequently when ladies send photos for a 'fit opinion' is that the shoulder seam is too wide. Identifying and visually marking your shoulder width is greatly enhanced by use of a couple of pieces of this flexible tubing.
Cut a piece of airline tubing about 15" long. Set it on your shoulder, wrap it underneath your armpit and I secured the ends together with a little black connector/coupling (also sold in the aquarium store).
It's a little hard to do this yourself, maneuvering it into place and getting it to stay put. You're best off doing this with a measuring buddy.
This tubing idea was shared with all of us at the April SFD Fit & Sew Retreat by Kay W. from Georgia. Thanks so much Kay; we all learn from one another.
There is also a video in the Sure-Fit Designs Learning Center that addresses shoulder length and how best to measure it. In this video, I use blue painter's tape. The concept is still the same, but I think the tubing is more accurate because it's narrower than the tape.