Another useful calculation is estimating a withholding shortfall. Suppose the total tax you owe for the year (based on simple example calculations) is $4,500, and your employer withheld $3,600 over the year. The difference is what you might owe at filing time (ignoring penalties or interest for this example). Understanding this subtraction helps you decide whether to adjust withholding via the W-4 during the year to avoid a surprise balance. Use the given numbers only for the example.