See I gotta push back a little on that. Knowing the numbers is what separates good welds from lucky ones especially when you're working to code. That old timer's advice works fine for stick welding in a shop but out in the field with X ray and bend tests riding on every joint you bet I'm checking my preheat temps. Had a job last year on some heavy wall chrome where my color was telling me 400 degrees but the contact thermometer said 550 and guess which one was right. Trusting the tool over my eyeball saved me from getting popped on a pressure test.