I think it was an Alex Anderson show once, she said for a quilt show everything has to be perfect and she has had to block one where she had it on a floor, (I presume it was waterproof) and it was almost sopping wet just trying to get it straightened up. I think the measuring tape is the device to use to help with squaring it. I have blocked needlework years ago and it worked for me. Just manipulate it while wet and fasten down to dry. My quilt teacher says sashes are also called coping strips....and one can sometimes manage to add/subtract dimensions there without undoing the center.