I think you might be referring to this post about the June Tailor Grid Marker:
http://www.quiltingboard.com/main-f1...t-t269358.html
It only marks one direction, so you'd need to pick it up, line up the horizontal line with your traced edge & then trace again. It's $30 at JAF without a coupon, so it's not terribly expensive. I've never used it, so I can't say how well it would work.
Personally, if I had a decent sum of money to buy something to square up various sized blocks, I'd buy the Karen Kay Buckley Perfect Adjustable Square ($75).
http://www.karenkaybuckley.com/store.php?cat=13