Jpet,
You should complete a W4 each year. Make sure you include any significant streams of income, including stock sales, number of dependents, and any tax credits. If your wife will be starting work include her income as well using the dual income chart. The process is much more time consuming than the old way but it is the only way to get a good estimate. The IRS tool can be found here:
https://www.irs.gov/individuals/tax-withholding-estimator
