Withdrawal method (drawdown rate)

Draw only what you spend (default): each year the engine draws exactly your retirement target (inflated for CPI). Your portfolio grows if markets outrun your spending.

4% fixed real: the engine draws the maximum of (your inflated retirement target, 4% of the starting retirement balance inflated for CPI) each year. The 4% acts as a floor, so if your real expenses exceed it the engine still pulls expenses.

Percentage of current balance (Bengen-VPW style): draw a fixed percentage of the balance at the start of each retirement year. Spending flexes with the portfolio: bigger years when the portfolio is up, restrained years when it's down. The engine recalculates the dynamic drawdown each year against the actual current balance, so the strategy self-adjusts to market conditions.

Some users prefer expenses-only because their lifestyle is stable and they want the portfolio to compound when markets are kind.

Some users prefer 4% fixed real because the conservative withdrawal floor has decades of historical data backing it (Trinity Study, etc.).

Some users prefer percentage-of-balance because they want spending to flex with their portfolio. Bigger years in good markets, restrained years in bad ones.

Expenses-only doesn't adapt to market conditions: a sequence of bad years still pulls the same dollar amount, accelerating depletion.

4% fixed real maintains a floor under spending. In years when your actual expenses are below 4%, the engine spends up to the floor anyway, which can leave you over-drawing in markets where the floor is unwarranted.

Percentage-of-balance removes sequence-of-returns risk but means lifestyle drops in bad markets, which can be psychologically harder to live with than the other two.