Before you make any changes, definitely try it with the waistband. You think this wouldn’t make a difference, but it does. If only the crotch curve in the back needs to be taken in, you can shave off a little of your pattern at the top of the crotch curve. Mark on the crotch curve, from the top, down to a measurement you want to take out. From this new mark, blend a new line to the original waistline before it gets to the side seam. Then make another muslin to test. I hope this helps 🙂