I came here to say that no, this is a bad bit of financial advice.

I'm also surprised to see your source. The book itself has a LOT more detail going into it.

For example, it's easy to justify eating a $20 lunch because that's just 1 hour of work. So you can justify it every day because you're working 7.5 or 10 hours a day or whatever.

That's not how it works.

That $20, you pay taxes on it.

Then you have your mandatory expenses -- rent, utilities, food. Childcare. Healthcare and medications. There might be lots of mandatory things.

After removing all of that, you might be making $5 an hour from that $20/h that was in your contract.

THAT is the figure that you need to be using for these "This $X is worth Y of my time". Not your gross income!

Using your gross income really only works for a PT job in high school when you have no expenses. A lot of people hear or come up with this when they're in that position, but it doesn't hold water when you're an adult with living expenses.