He didn't come up with it. It's been around for a long time. The formula depends on light spectrum. If done right it is fairly accurate. It's all known math after all.
The problem of exact accuracy is that the advertised spectrum is not exact, so unless you know the exact kelvin of your light, it will be a bit off. But definitely close enough to tell if you're in the right range.
There are websites that can do the calculations for you. But PAR isnt needed. Just use lux or lumens, they are good enough. Unless you're in a lab doing science, there is no reason to be that accurate with your light.
