When I chose to take an early distribution on my 401k-of $3090.19; I withheld an est amt of $618.04-why does turbo tax say I owe addtl $309 if I should pay 10%?

The plan administrator is required to withhold 20% of the distribution from the 401(k) for federal income taxes.  The taxes withheld are entered on your federal tax return as a tax payment.  The distribution is entered on your tax return as ordinary income along with your other income and taxed at your current tax rate.

Since this was an early distribution there is a 10% early distribution penalty assessed on your federal tax return and entered as a tax liability.

If your total tax payments are greater than your total tax liabilities you will receive a tax refund for the overpayment.  If your total tax payments are less than your total tax liabilities then you will have taxes owed.