These tables were derived from Publication 15, Circular E, Employer's Tax Guide
from the IRS. If you know the total amount you want to withhold, you can subtract the amount which will be withheld
based on marital status and number of exemptions to give an additional withholding amount. If these tables change in subsequent years you will be at least
partially protected if you forget or are unable to change your withholding preference.
All calculations are done at the annual level (allowable per above publication) and may not be exact due to roundoff error.
The following example is from that publication and reproduces its final answer:
Annual income tax withholding. Figure the income tax to withhold on annual wages under the Percentage Method for an annual payroll period. Then prorate the tax back to the payroll period. Example. A married person claims four withholding allowances. She is paid $1,000 a week. Multiply the weekly wages by 52 weeks to figure the annual wage of $52,000. Subtract $14,600 (the value of four withholding allowances for 2010) for a balance of $37,400. Using the table for the annual payroll period on page 40, $3,010 is withheld. Divide the annual tax by 52. The weekly income tax to withhold is $57.88.
If you are experimenting with your annual estimated from all sources, you may enter it in the "Salary or Pension" field, but then check "No Multiply".This table is for 2010 and is for reference only.