Use conservative assumptions
Model current invested assets, annual contributions, years until retirement, expected return, inflation, and retirement spending. Keep nominal returns and inflation consistent rather than mixing today’s dollars with future dollars.
Test a lower-return case, a higher-spending case, and a delayed-retirement case. The range is more useful than a single optimistic number.
Focus on controllable inputs
Savings rate, fees, asset allocation, and retirement date are easier to influence than market returns. Revisit the plan yearly and after major income, family, or health changes.