Sorry if I’m missing something, but… what’s the problem with that quote? That’s a widely-used heuristic that helps to estimate doubling times without using a calculator (see [the Wikipedia entry](https://en.m.wikipedia.org/wiki/Rule_of_72).

He does walk the reader through a lot of “back of the napkin” math, in order to help the reader get an intuitive sense of the models he’s using. But my impression overall is that he backs those hand-wavey calculations up with more serious calculations throughout the book.