Deep breath ... here I go on the princess seam FBA idea! Since Simplicity 2566 also has a jumper that uses the same four bodice pieces as the vest .... and I'm still debating the idea of doing this jumper. I first had to decide how to handle the cutting lines. I decided if I do make the jumper, I will just tape the bottom back on ... so I cut along the vest's cutting line carefully and folded the rest up neatly.
Then, I held up the front piece to myself to find *my* bust apex and marked it, then lined up the side piece and marked the apex out in the side. Subtract the amount on the front panel ... and (I think) that should give me how far out I need to extend the side piece, minus seam allowance. Here's what I have so far:
Simplicity seems to believe that someone with size 18 shoulders ought to have a 40 inch bust ... mine wibble-wobbles between 42-43 inches depending on water retention. Of course, Simplicity's measurement chart also says I should have a 32 inch waist while mine varies between 34.5 and 36 inches - again depending on water retention - so I may need to add to the waistline, which I had to mark on the side piece. Oddly, there is no finished garment measurement for the waistline, although the bustline finished measurement is printed on both the pattern and the envelope.
If anyone more experienced at a princess seamed FBA sees a major mistake here, please post up a correction! Meanwhile, I should root around for some lunch, then pull out my linen/cotton to start this experiment on the intended lining fabric.