I've got an opportunity to buy a used 72" Starrett Ruler C416R. I primarily want to use it to align my "new to me" 12 inch SCMI jointer (also a craiglsist find!). So here's the challenge - how do I ensure a the ruler is actually straight (within tolerances reasonable for this use)? I have a 12 inch Starrett square and a 24 inch straightedge from Woodcraft. I won't be able to bring it back to my shop without purchasing it and I don't want to joint a reference board on a jointer that I'm not sure is straight. Thanks in advance for your thoughts!