If you have measured across the centre of the quilt in both directions to get the measurement, and then is still hasn't eased in to sit flatly, I think I would make a judgement as to whether the quilting could flatten it. If not, I would 'fudge it a bit by easing in all of the seams along that section and don't trim anything until it is flat.Most frustrating when the measurement might only be out one sixteenth of an inch. Good luck with the fixing, as it is a nice , dramatic quilt.