Old school trick, but it works. A chalk line is one of those simple tools that gets forgotten now. It beats trying to stretch a string or eyeball it for sure. That straight line makes setting posts go twice as fast.
Spot the same thing happening everywhere with basic tools and common sense. We get so used to complicated solutions we forget the simple fix right in front of us. It's like we're all trained to look for the high-tech answer first. Makes you wonder what else we're overcomplicating for no good reason.