Tartan,
I can see the issue in your picture. Try this for next time.
Put the ruler over the quilt to square up. Tug the quilt out the line you want. I use the point of a seam ripper or stiletto and gently tug the edge into shape. I can usually pull the quilt into something fairly reasonable as there is a little stretch.
I don't need to wet the quilt but that might help too.
Good luck!