Hi tko1020! There are calculations for figuring the underpayment penalty. You should probably use the IRS Tax Withholding Estimator to determine how to adjust your W-4. It will allow you to enter the amount of wages you've already earned for the year. It will project how many additional paychecks you will have until the end of the year. Here's a link to the tool:
https://www.irs.gov/individuals/tax-withholding-estimator
If you feel like this will not be enough of a change with your withholding, you can check out the Estimated Tax worksheet in the Form 1040-ES instruction booklet. This will help you determine if you still don't have enough withholding.
https://www.irs.gov/pub/irs-pdf/f1040es.pdf
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