Accumulate 25 times your expected spending.

The link is to a website that was free, and provided some great financial planning. Looks like a redirect when I clicked on the bookmark so no guarantee of what it will actually provide

https://maxifiplanner.com/esplanner-basic-now-maxifi

Market Watch article about a financial book that's very valuable for your planning, "The 7 Most Important Equations for Your Retirement"
Likely free at your local library

https://www.marketwatch.com/story/7-...ent-2012-06-27