First Muslin

After posting yesterday about Simplicity 3788 possibly being suitable for a smocked project, I was inspired to go ahead and make a muslin to get started on the project. Let me just start by saying that this top is easy! There are only six pieces including the straps and ties. It went together quickly and the instructions were very clear.
Having said all that, the first muslin did not work. I am however, still excited about the project and very optimistic. I cut a size 20 straight from the envelope. I usually start with an 18 and do a FBA, but I figured this top has no shoulders, and my full bust measurement matched the bust measurement of the 20, so I decided to just sew it up and see. It was way too big. There was gaping under the arms and in back, and the straps were too far apart in back. The front was okay, but I think I could use a smidge more length to go over the bust.
So it seems I will have to use my high bust measurement and do a FBA anyway. I measured myself again last night just to be on the safe side, and my high bust measurement has gone down! I can start with a 16 instead of an 18. I've been seriously dieting for the past month, and it has begun to pay off! So I will cut a 16. The back of the top is all one piece. Instead of trimming at the side seams, I think I will move the fold line in a corresponding distance and see if that will bring the straps in to the right place. Interestingly, the center front pieces are the same for all sizes except the top section varies slightly in length among sizes. I haven't decided yet, but I may need to add a tiny bit of length overall to the top. I'm very long through the body and I want to make sure the top covers everything and looks stylish.
I'm thinking this top could work well for the smocked version I have in mind. The only issue I can forsee right now is finishing the top edge and covering the seams around the insert with the lining. For now I'm looking at it as a challenge, not a problem.