To avoid underpayment penalties, the IRS wants you to have paid or withheld within $1,000 of the tax due by the end of the year (although there are several exceptions).
If you save the money in the bank and pay when you file, you may be hit with an underpayment penalty.
Or you can increase the withholding on your husband's salary to cover the shortfall.
Or you could increase the federal withholding on your 1099-R to cover the shortfall (although that doesn't seem fair, does it?).
Or you can just send the IRS the 1040-ES vouchers yourself on a quarterly basis to pay for the expected taxes on Social Security so that you are close to even at the end of the year.
See this link - https://www.irs.gov/uac/form-1040-es-estimated-tax-for-individuals-1 - to see the instructions and the forms (you can print and mail them).
Don't worry about the complicated instructions - they are there to help you estimate your income and tax in a variable situation. But if you know that you want to make 4 payments of $1,500 apiece to cover that $6,000 shortfall, just enter your name and address and SS number and the $1,500, write the check, and mail it.
