The basic IRS withholding tables are based on the assumption that the paycheck is the employee's sole source of income. Obviously the true tax bracket could be much higher based on several possible factors:
- more than one job
- spouse with another job or jobs
- other taxable income: investments, rentals, etc...
To minimize the need for the employee having to make quarterly estimated tax payments, line 6 of the W-4 form, which the employee uses to inform the employer of his/her withholding situation states "Additional amount , if any,you want withheld from each paycheck."
When you set up an employee in the payroll module, look at the screen into which you enter the employee's marital status and withholding allowances. There's another box which is labeled (in QB 2008 Pro) as "Extra Withholding". So you can't specify a rate, but you can boost the withholding this way.
Case closed?