All you need is a large piece of paper, and your rotary ruler.
Tape copy paper or whatever you have on hand together. Measure this to your 20 1/2".
Center this over your block and use your ruler to trim around the paper. You can fold the paper in 1/4's and use those lines to match up to your blocks, or what ever other lines you may need.
You can also fold the outside edges in at a 1/4" to make sure you aren't cutting off your outer points if you have any :wink: :D:D:D