If you have made a toile of a princess seam bodice and you notice that you have gaping at the armscye, you want to adjust the pattern so that the bodice fit your body better. Below I will show you how to fix and remove the armscye gaping.
HOW MUCH FABRIC TO REMOVE FROM THE ARMSCYE
When you are wearing the toile, pin the excess fabric (gaping) in the armscye so that the armscye sits nicely on your body. Measure how much fabric you have pinned away. This is how much you will remove from the armscye on the pattern.
ADJUST THE ARMSCYE ON THE PATTERN
On the side front bodice piece, draw lines to mark the seam line at the princess seam and armscye. Then draw a straight line from the bust apex to the armscye (where you pinned out the excess fabric).
From the armscye seam line, measure the amount that you pinned out from the armscye to remove the gaping. Draw a second line from the bust apex seam line to the armscye seam line.
Cut one of the lines from the armscye to the bust apex seam line. At the apex clip to the seam line, leaving a hinge so you can rotate the pattern.
Close the dart you have marked by rotating the pattern at the hinge. The two lines you previously drew should now be in the same place.
Place paper underneath the armscye and redraw the armscye into a smooth curve. Do the same for the opening at the princess seam where you cut into the pattern to leave the hinge.
Cut out the pattern piece.
Sew another toile to check the fit, or go straight to your good fabric if you feel confident about the adjustment!