Estimate Future PE for a company using only earning/equity growth rate

I’m reading a book called Rule #1: The Simple Strategy for Successful Investing in Only 15 Minutes a Week! by Phil Town.

Inside the book, he claimed this is the easy formula to calculate the future (10 year) PE for a stock.

Just double the digits Earning or Equity Growth rate.

For example: if the growth rate is 8%, then the Future (10 year) PE is 16.

Does anybody know how he came to this kinda of formula?