To knit your adjusted sweater, cast on the total width you calculated, work until you are about 2 inches below the armhole. work the short rows you need in the front, continue to the armhole. Bind off the stitches in section B. Begin to decrease one stitch at the beginning of each row until you are at the width of A. Work up straight until you have achieved your armhole depth. Work your desired shoulder finish.

I have not mentioned the neckline finish. That is done the usual way with those infamous words......at the same time.