Recently I had a longarmer quilt a Quilts of Valor medium size quilt and it came back very distorted among other issues.. I squared it up the best I could and tried to think of an easy way to make the sides match. When I sewed the binding on the first long side, leaving a two foot length hanging off the end, I then measured the length and noted it for when I sewed the matching side. Once again, after mitering the corner and sewing the short side, I measured and wrote it down. Before sewing the next long side I measured from the mitered corner down the length of binding, put a pin in place, and matched the end, middle, and 1/4 marks applying the binding and then repeating the same for the final short side. Note that the two foot length of tail left in the beginning has to be factored it on the final side. This worked great and the quilt is now only a fraction of an inch off in length and width. Unfortunately, due to the severe distortion I did lose some of the points in the pieced border but if you don't have a pieced border this would work even better.
Very good tip White Eagle. Hope that I will keep my quilts straight and won't have to modify my plans at the end. That must have been frustrating to see your quilt coming back distorted from the way you sent it.
Angèle from NWO