There is also the whole formula set that adequately describes the phenomenon. It is a three dimensional set of differential equations, where you can only ever know five out of six starting conditions, so you need to iteratively adjust the sixth one until your error term is small enough. The formula set was developed 80 years after Johnsons proposal, using the advancements in computing technology, but the results are not better than what we get with the Bill Factor
So we thank Bill Johnson every day we use his Factor.