I am withdrawing all of my 401k. 20% is taken out already and I will put aside 10% tor tax time. How can I find out how much I will be charged above the 20%?

All withdrawal is to be reported as ordinary income on your tax return. This withdrawal is also subject to 10% early withdrawal penalty unless you qualify for an exception.
So how much tax will be charged depends upon your other income and the amount you have withdrawn. Read Your U.S. Tax Return: Elective Deferrals 401(k) Plans

Expect to pay at least 25% of the distribution on taxes. It could be as high as 50%, depending on your overall income level.

The norm would be about 30-35%, spread out as listed below:

- 10% for the Early Withdrawal Penalty (which CANNOT be mitigated; that means that even if all deductions and credits factored in reduces your total income and tax liability to zero, you will STILL pay the 10% Early Withdrawal Penalty).

- 15 - 25% for federal income taxes.

- 1 - 5% for state income taxes.