Just found out why my patio joints kept cracking, it was the saw timing

I was watching a DOT crew pour a slab off I-35 last Tuesday and the finisher told me they wait until the concrete hits a certain hardness before they run the saw. I always thought you just cut as soon as you could walk on it, so my 400 sq ft patio in Round Rock had hairline cracks show up within 2 weeks. He said if you cut too early the edges tear out, cut too late and the stress already found a release point. I checked my pour log and I was sawing at like 4 hours when I should've waited closer to 8 or 9 given the heat. Now I'm testing a moisture meter to judge the window instead of guessing by feel. Has anyone else had better luck using a specific psi reading or do you just go by thumb pressure?