Is there a handy formula for determining the length of shock cord you need for an uq? I will be making an uq that measures 48" x 76", with shock cord in channels sewn around all edges. If the cord were totally slack, I'd need just under 21 feet. But of course the cord won't be slack, so I would need less cord - but how much less? And then I know I'll need to add some cord back in for attaching the quilt to the ridgeline, but again, I'm not sure how much. Any help would be appreciated!