You can avoid an underpayment penalty if withholding or
estimated payments equal at least 90 % of your
tax liability for the current year, or 100 % of your
tax liability for the previous year (or 110 % if your income was more than $ 150,000 for singles and married
joint filers).
If the pro forma
joint return results in a balance due and this is unsatisfactory to you, you can increase your withholding by filing new Forms W - 4 or by making
estimated tax payments to cover the expected balance due.