The Social Security benefits are taxable based on the amount of other income entered on your tax return. TurboTax does this automatically. There is no user input on the taxable amount of the SS benefits.
Up to 85% of Social Security Retirement/Disability/Survivors benefits becomes taxable when all your other income plus 1/2 your social security reaches:
Married Filing Jointly - $32,000
Single or Head of Household - $25,000
Married Filing Separately - 0