The new deduction is considered "below-the-line," which does not reduce your adjusted gross income but lowers your taxable income. You will fill out your Schedule C as normal -- listing your income and subtracting your qualified expenses and then calculating your self-employment tax.

The deduction will come off somewhere on the 1040 form against your business income, and there will likely be some kind of worksheet to walk you through the calculations.

More guidance should be available by the summer, said Zaino.

Estimating Your Tax Burden

To figure out your first quarterly payments, you most importantly need to know your tax burden in 2017. One way to skirt the new math is to pay what you paid last year - even if you think you will owe less in 2018 -- so that you avoid penalties for underpayment. Or you can pay 90 percent of what you think you will owe in the current year.

You can also "annualize" your quarterly payments, said Lisa Greene-Lewis, a CPA and tax expert with TurboTax, which means that you base your payments on your income for the whole year, instead of big fluctuations month-to-month.

TurboTax has a free TaxCaster app that will take your 2017 info and calculate your tax burden for 2018. It takes into account your pass-through deduction, if you had freelance income previously. For spreadsheet pros, there are several online templates that attempt to work out the 199A pass-through deduction, which you can search out.

If you employ a tax professional, they are probably engaged at the moment in a review of all of their client files and can help you figure out the right amount to pay.

Tax accountant and financial planner Phyllis Jo Kubey, who is based in New York and specializes in artists, is not only looking at freelance income but also at making sure withholding is correct on W-2 paychecks. Most of her clients have a mix of self employment and wage income.

Kubey is not yet sure what to do with her clients who have income from sources outside of the United States because the law specifically refers to "domestic income," and the IRS has not clarified what that means.