Child support is calculated using the adjusted gross income. This is your actual income after all relevant taxes are taken out. You should also make sure that any municipal taxes you pay are being accounted for as well; these do not show up on a W-2.
First, "imputing" may be the wrong term. That's what the court does to come up with an income figure if you are voluntarily unemployed or underemployed.
Second, I disagree that your tax return's adjusted gross income is the correct figure. Washington court decisions make it clear that some deductions that are appropriate for federal income tax purposes are NOT appropriate when calculating income for child support. See, for example, Marriage of Mull.
In my experience, most lawyers and courts use the largest income number shown on the W-2, which is probably box 6. Box 1 shows income after deducting all retirement contributions, but only $5,000 a year in voluntary retirement contributions can be deducted when calculating income for child support purposes under Washington law. That is done when filling out the support worksheets, at line 2(f).
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