Your taxable social security benefits would be calculated the same as if you were a resident of the US. US citizens are taxed on their worldwide income.
However, if you live in one of the following countries, your Social Security Benefits are exempt from tax on your Form 1040:
Canada, Egypt, Germany, Ireland, Israel, Italy (if you are also a citizen of Italy), Romania and the United Kingdom.
This information is taken from IRS Publication 915 (See the screenshot below)
If you meet the qualifications for your Social Security Benefits to not be taxed, report the Social Security exactly as it appears on your SSA-1099.
To deduct the taxable amount of your Social Security Benefits you need to:
On the Wages & Income tab, scroll to Less Common Income and click on Show more
Click the button beside Miscellaneous Income, 1099-A, 1099-C
Click the button beside Other Reportable Income
Click Yes on the "Any Other Taxable Income?" page
Enter a description (IRS PUB 915 - SS Income Not Taxable) and the amount of Taxable Social Security as a negative number
Click Continue then Done
To look at the Form 1040 to confirm that amount has been deducted on Line 21 (and to double check the taxable social security amount), please follow the instructions below.
Please click on Tax Tools in the column on the left side of your Turbo Tax screen.
Then click on Tools; (See the second screenshot below)
Then View Tax Summary (See the third screenshot below)
Click on "Preview my 1040" in the left column. (See the fourth screenshot below)
If you want to confirm the taxable amount of your Social Security Benefits, Click hereto access a Social Security Benefits Worksheet. You can work through the calculation to see if you should have taxable social security.
[Edited 4/9/18 | 5:58 am PST - changed content of original answer - some SS not taxable to residents of certain foreign countries]
Under IRS Pub 915 (page 5), SSB's paid to U.S. citizens who are resident of certain countries (including UK) are exempt from U.S. tax on their benefits (only know this as I reported it as taxable income once and the IRS changed it). How can I have TurboTax include the benefits in box 20a on 1040, but have $0 in box 20b?