futurefin-fire-domain-reference

The FIRE number equals the grossed-up annual need divided by the safe withdrawal rate, plus a debt term. The need comes from one of three modes: manual amount, annual retirement expense minus persistent income, or current income replacement, each inflated per month.

The gross-up converts the net annual amount needed into a larger gross withdrawal using a closed-form walk through Spanish IRPF brackets, scaled by the taxable gain ratio. It replaced a 90-iteration binary search with identical results within 0.01 euros.

The target grows because the base amount is multiplied by an inflation factor each month, while a decaying debt term is added. With zero inflation and live debt the target strictly decreases, so it is no longer monotonic.

Yes, parity is enforced through the shared fire-parity.json fixture and the same closed-form gross-up on both sides. In transaction-derived savings modes the preview must consume the summary's effective equivalents rather than recomputing from the budget.

Do not use it as a bug triage runbook, to change the economic model, or for environment and configuration questions. Those belong to the debugging playbook, change-control, and config-and-flags skills respectively.