Leave a margin of safety

Estimates are flattering. Reality is not obliged to match your spreadsheet.

A margin of safety is the gap between what you think will happen and what you can survive if you are wrong. In money, it is cash you do not invest. In projects, it is time you do not promise. In health, it is sleep you do not trade for one more hour of output.

The people who look conservative in good years are often the only ones still making decisions in bad ones.