geez, I measure the "fall" and then across the bed, then add the "fall" again...
Then for the length, from the top edge of the bed to where I want it to end at the foot.
FWIW I also get batting on the roll, so I can afford to make it how wide I want, without consequence. If you're putting it on a frame, I would scoot the batting as close to the edge of your work as possible, because once you quilt there it's gonna stay put, kwim?