Unlike the widespread method for bust adjustments on princess seams where full bust and small bust adjustments follow different steps, our method can be used both ways. The SBA will thus use the same steps as the FBA, except that instead of adding width we will be removing some.
To know by how much you need to reduce your bodice, start by making a muslin of the dress. To have accurate measures, make sure to make this muslin in a fabric of similar weight, elasticity and recovery as your main fabric, as these factors will influence the fit of your dress. Don’t forget to attach the skirt, as it will weigh down on the bodice and change the fit as well.
To measure your adjustment value, pinch the fabric vertically at the bust apex over both princess seams. Pin the excess fabric on each side, making sure that the side seams are straight and are not pulled to the front. Measure the depth of each pleat and multiply it by two (for the two layers of fabric of the pleat) and make the sum of both to find your total adjustment value. For instance, if you have pinched ¾” on each side (forming two pleats ⅜” deep), you total adjustment value will be 1½”.
We will start by modifying the side front (piece #3). Trace the seam lines all around the pattern piece, 3/8” away from the edge, to use to draw your slash lines. Draw three lines on your pattern piece:
Slash along these three lines, leaving two pivot points: one at the armhole and one at the waist seam.
Place your piece on a new sheet of tracing paper, and draw a horizontal line inwards starting from the bust apex. Place a point on this line at a distance of 1/4th of your adjustment value. This is the new bust apex. Pivot the two free parts of the pattern piece so that the slash lines pass through the new apex (c.f. figure) and tape in place.
Measure the overlap between these two parts and write down this value (measure A): we will need it to modify the center front. Re-draw the bust curve, and place the new bust apex notch on the same horizontal line as the previous one.
We will also be removing width from the center front (piece #1 or #2). Just like with the side front, use the seam lines to draw three lines:
Slash along these lines leaving a pivot point at the armhole and another at the waist seam.
Place your pattern piece on a new paper sheet and draw the point corresponding to the new apex. Again, it is placed on the same horizontal line as the old one, at a distance corresponding to your adjustment value divided by 4. Pivot the two free parts so that the apex is in line with the slashed lines and tape in place. Measure the distance between these two parts (measure B).
We now need to change the height of this piece in order to make sure that the princess seam lines are the same length. Draw an horizontal line passing through the bust apex and slash through this line. Overlap the two parts on a distance corresponding to A + B. Redraw the bust curve. The bust notch is placed on the horizontal slash line of the bottom part of the pattern piece.