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The federal tax withholding is not based on a specific percent.
It is based on how the W-4 form is filled and the tax withholding tables.
see W-4 form here - http://www.irs.gov/pub/irs-pdf/fw4.pdf
and tax tables - http://www.irs.gov/pub/irs-pdf/n1036.pdf
You also may use this withholding calculator - http://www.irs.gov/individuals/page/0,,id=14806,00.html
To determine if you will owe any taxes at the tax time - we need to estimate your tax liability based on your income, deductions and credits and adjust withholding accordingly.
Let me know if you need any help.