Your Questions on Paying Estimated Taxes, Answered

Q: In December, I determined I had enough cash available to pay taxes owed on a lump sum, one-time Roth conversion of $200,000 and sent a check for taxes using the estimated-payment form. How do I avoid an underpayment penalty? At the end of the year, I convert to Roth but only if I have the available cash to send a check for the taxes.—a reader in Somerville, N.J.