The simplest way to do this is to look at your prior year tax returns and determine the percentage of your social security that was taxable those years. Per IRS rules, you will either pay tax on 0% of your social security benefits, 50% of your benefits, or 85% of your benefits.
If your 2017 tax return shows that 50% of your benefits were taxable, use only half of your 2018 benefits when using the IRS calculator. If 85% was taxable last year, use 85% of your 2018 benefits. You may want to check 2016 and 2015 percentages as well, just in case the 2017 tax year was an anomaly. Best of luck!