The Perfect Fit there is a princess FBA method that doesn't add width, so I decided to give it a try.
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In back, I did a 1/4 inch broad back adjustment.
For muslin #2, I shortened the back waist by 1/2 inch, tapering to nothing at the side seams (basically a sway back alteration), and I added 1/2 inch of width at center back, both in the bodice, as well as the waistband and skirt. Then I added back the 1/2 inch I took out of the bodice to the top of the skirt. Instead of gathers at the upper bodice, I did an inverted pleat.