If you think your income will approximate 2017, use those figures. Otherwise, estimate as best you can.
You may be able to get an IRS Wage an Income Transcript.
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Wage and Income Transcript - shows data from information returns we receive such as Forms W-2, 1099, 1098 and Form 5498, IRA Contribution Information.
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Current tax year information may not be complete until July. This transcript is available for up to 10 prior years using Get Transcript Online or Form 4506-T.
