Also remember that the 20% federal withholding is simply the mandatory amount, it is not the amount of tax you will owe. Your income tax is only calculated 1 time on your tax return. Your tax rate might be 15%, in which case too much federal tax was withheld and you will get a refund, or your federal tax might be 22%, 24% or higher, in which case you will owe an additional payment.
Likewise with your SC tax, if you have 7% withheld and the tax turns out to be less, the extra will included in your tax refund.
