I like it and it's something that I do often in my head for such big numbers. I had not thought of writing it down though. I did write down some shortcuts on multiplying with 5's. You can see all 3 sections on my site.
https://steemit.com/education/@johnwjr7/fun-with-5-s-1
https://steemit.com/education/@johnwjr7/fun-with-5-s-2
https://steemit.com/education/@johnwjr7/fun-with-5-s-3

Please let me know what you think of those.