Prism99 is correct. you need to block the quilt, and starch it as much as possible. The fabric needs to be as stable as possible to keep the edges as stable as possible. If it comes out feeling like cardboard, great! that's perfect. Starch will wash, right now you need stiff as a board. Once you have starched and blocked it. Measure the depth of each white along the outer border. Measure from the point to the center of the outer edge of each block. When you have found the smallest one along each side, use that measurement to trim your sides.Check the full size of an existing block in the middle, and cut it in half. that measurement is what the half blocks should be. But done be alarmed if you have to make your half blocks a little smaller to square it up. that measurement is just a guide so to help you get a feel for how much you are cutting off your quilt sides. Also, don't "Iron" your quilt. that will push it out of shape. Always press it. Spray starch an area and press until dry by lifting up and down, never slide side to side. Don't panic that quilt looks beautiful, and, unless you have balloons , a good long arm quilter will be able to disguise any bubbles. I am a long arm quilter, and I just had a wedding ring quilt that looks like bazooka bubble gum. And it turned out gorgeous.