I really don't know what has caused your corner to be off but I was wondering if it could be due to how you squared up the flower block. Did you use a ruler with the diagonal 45 degree line on it and put that line right down the center of that sewn line in the middle of the flower before cutting? Your block could be square but not "squared" on the flower. Not sure if that makes sense to you but hopefully you will get it figured out. And as someone else mentioned, Jinny Byer does hand piece and that does make it easier to be more accurate. By the way, what a lovely block that is!