I made a swing jacket and every time I put it on the sides hang longer than the middle of the back. I find this really annoying so next time I make something this shape I want to adjust the pattern so that the hem will hang reasonably straight.
Why is this happening? Simply because the side seam is longer than the centre back and it doesn’t sit out as a triangle like it does in the pattern diagrams once you put it on.
So how do you calculate the short rows you need to add? First you need to work out how wide it is going to sit compared to the width of the hem. My theory is that this will be your width at the hem.
Next you need to work out where to put the short rows. My current plan is you put them just above the hem.
The number of short rows will be determined mathematically by comparing the 2 triangles; the pattern shape and your shape and working out how deep the red section will be. Your gauge will then determine how many rows you need. Once you know the number of rows you can work out how many stitches to include in each short row to get a relatively straight line.