I love this. I’m a big believer in the utility of making back of the envelope calculations. It’s amazing how often it can reveal poor assumptions or that someone is trying to peddle malarkey.
The technique is also extremely powerful. For example, the slides at the link pose the question: “How many piano tuners are there is New York City?” At first it seems like it’s impossible to know or that at least you’d need specialized knowledge but that’s not true. By making some reasonable assumptions—the average piano is tuned once a year, for example—and combining them with a plausible estimate for the number of families in New York, you can reach a reasonable guess for the number of piano tuners. The idea is to get an estimate that is accurate to within an order of magnitude.
If nothing else, doing some of the exercise on the slides may help you with your next job interview when someone asks you how many pizza parlors there are in San Francisco.