For filing the 2025 return, you do not attach your spreadsheets or bank statements to your Form 1040. You simply enter the total cost basis you calculated into TurboTax. The transaction will be reported on Form 8949 with Box E or Box F checked (indicating the basis was not reported to the IRS).
If the IRS ever questions the numbers (an audit), they will expect a contemporaneous record. You should keep a file (digital or paper) containing a clear summary showing the date and amount of every reinvested dividend, the First and Last Statements to prove the initial purchase date and the final liquidation, and a few samples from the 20-year span to prove the dividend reinvestment plan was active.
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