How accurate is the hand technique for grilling?
While I suspect that the formula is mostly reliable if you repeat the experiment with the same person, I found the individual heat sensitivity varies a lot. Most professional cooks and seasoned homemakers can handle (pun intended) higher heat than those who rarely expose their hands to high temperatures. In our home, we’ve had heated discussions about whether the dishes in the just finished dishwasher are too hot to touch or not. And this is just one example.

If you exclude the differences between individuals, using your own hand’s heat sensitivity to gauge a temperature range is absolutely possible (as long as a few degrees don’t matter). It takes time and practice - which is usually described as “experience”. I personally don’t think in “degrees” though, but more in categories like “too cold” / “should be ok” / “too hot” and a few steps in between. 

So in short, if you need a precise temperature, get a thermometer. If you are still working on judging by hand, either get a thermometer or plan a few trial-and-error loops. And sometimes the exact temperature is not important for a recipe to work.

Using the X-time at Y-distance formula may work, if your heat sensitivity is similar to the author’s.