I always figured a speed square was good enough for setting up small slabs. Then I watched a video from a guy named Mike out of Kansas City who showed how being even a half inch off on a 12 foot form throws everything out of whack. He laid out a 3 4 5 triangle with string and stakes, and the corner came out dead nuts square. I tried it on a 4x4 patio pad last weekend, and my diagonal measurements matched to within 1/16 of an inch. Way easier than eyeballing it with a framing square too. Has anyone else here switched from squares to strings for bigger pours?