My FIRE number started at $1.2 million, then moved to $1.5 million, and is now at $1.8 million. Each change taught me something.

$1.2 million was based on lean spending estimates that didn't account for healthcare, home maintenance, car replacement, or any real buffer. That number would have left me perpetually stressed about money.

$1.5 million added a more realistic healthcare estimate (~$12,000/year until Medicare) and a home maintenance reserve. Better, but still tight.

$1.8 million reflects what I actually spend plus a margin for the unexpected, calculated as 25x my real annual expenses of $72,000. It means working a few more years than my original plan, but it also means actually being financially secure rather than optimistically broke.

The lesson: be honest about what you actually spend before you set your FIRE number. The spreadsheet only works if the inputs are real.