Estimated-tax / §6654 safe-harbor + underpayment-penalty estimate

Evaluates the §6654 estimated-tax safe harbors for an individual and estimates the underpayment penalty if the safe harbor is missed — for anyone with lumpy or under-withheld income (equity comp, business owners, Roth converters). The required annual payment is the lesser of 90% of the current year's tax or the prior-year safe harbor (100% of last year's tax, or 110% when prior-year AGI exceeds $150,000 / $75,000 MFS). No penalty applies if the balance due after withholding is under $1,000, or the prior year had zero tax. The penalty is estimated with the regular method (four equal installments, withholding credited evenly, overpayments carried forward) at the IRS underpayment rate — 7% for 2025 and 2026 by default, overridable per request since the penalty accrues into a year whose quarterly rates are unpublished. Deterministic; not tax advice.