I actually borrowed it from Nick Soulis, who I believe got it from a friend. You can find the original doc in his Excellence Project post, which is a ranking of the all-time greatest teams. It's easy to find and is in the first few posts. This
link will take you to it or you can find it in the dynasty reports.
I just modified the formula in the spreadsheet to accommodate what I needed with a shorter season. But the formula is mostly the same. I tweaked the bonus points in the playoffs and keep the bonus points in my calculations for preseason rankings. I actually rank all of the teams with the spreadsheet, but generally only report the Top 25.
Thanks for reading!