I don't mean to confuse you, but this might. I would measure the star blocks and determine which seam caused the offense. Then take out that seam, move it over just enough to get the 12 1/2 you need. I would number the star blocks and you might find some that don't need to be "let out".
On my machine are 5 needle settings. Sometimes just sewing them on a narrower needle setting will do it. Best wishes. I wish I was there, I would help you.