[RantyDave]

I get completely different numbers (although the tldr is no less alarming). Does someone want to check my work? https://gist.github.com/RantyDave/41ac164cfcf527a91d3b97c8dd...

[shapefrog]

In your print statement ... (berkshire_return/100)*(1/10)-1:.4f ... gives the annualised return (assuming start with 100 and 10 years).

That brings you into alignment with the article.