Put on your muslin dress
Pin away the extra fabric on you lower back
Mark with a pencil on both sides of your alteration, where you have put in your pins. Take out the pins and draw between your pencil markings. Now you can measure how much you need to adjust and remove from your pattern.
1. You need to work with your back pieces. Here I show how to do it on version 2. You can use the same alteration on version 1. However, on version 1. there is no darts on upper back.
Trace all the strokes from your test dress on to your pattern. Do it by measuring on the test dress and copy the shape and measurement on to your pattern. The red strokes is the new ones.
Cut into your pattern where your red strokes are.
2. The measurement on the top and bottom must be the same
Throw the small pieces away
Now you need to measure the waistline on both your bottom and top, to be sure that they have the same measurement. You need to measure on both sides of your darts.