I don'the see how your longarmer even accepted your quilt. Is she aware that the top and back are exactly the same size?
I always insist that the backing be 6" longer and 6" wider than the top to allow for pinning to the rollers and side clamps without interference while quilting. You can work with less if necessary and everything is perfectly square but when both are exactly the same it is impossible to load the quilt properly.