What if you increased the seam alliance on the seam where the indented edge comes together? You could measure the "extra" & divide it by 8 (the number of indented points) & maybe subtract a smudge from the size you need to increase.It might even need to be graduated from the outside to the center (like a dart) since it appears that there might be more excess on the outside edge than in the center. I would also be gentle because of the bias edges, and don't uuse short stitches or it might pucker. Good luck & let us know what happens!