Curse you for planting this idea in my head. I was at a tour of Drexel with my son today and their student run investment fund turned 250k in 2007 into $5M today, a 20x return. So not I only did I have to do this in my head (and make my son follow along) but I had to do it a different way, because well you know. I got just over 4 doublings in 17yrs or a double every 4.25 years so I just divided 72 by 4.25 to get ~17%. Close enough to the actual 23% for monkey math but not as accurate as your method.
Curse you for planting this idea in my head. I was at a tour of Drexel with my son today and their student run investment fund turned 250k in 2007 into $5M today, a 20x return. So not I only did I have to do this in my head (and make my son follow along) but I had to do it a different way, because well you know. I got just over 4 doublings in 17yrs or a double every 4.25 years so I just divided 72 by 4.25 to get ~17%. Close enough to the actual 23% for monkey math but not as accurate as your method.