If you had federal income tax withheld from your social security benefits, you enter that withholding on your tax return when you enter the SSA-1099. You are taxed on up to 85% of your social security income at your marginal tax rate whatever that rate is. If it's 12%, that is the rate you will be taxed on up to 85% of your social security income. You will get a credit on your tax return for the 7% withholding which is on 100% of your social security income.
