If for 2018 taxes:
The pre-withholding is not a stand-alone assessment. Your 2018 tax return will be the final accounting of everything for the entire year. The 20% plus 10% prepayment is just an estimate of what might be required for the tax year.
When you eventually prepare the year's tax forms, the 30% withholding gets applied and added to ALL your other tax withholding for the year as a total credit. Then all your income from all sources is added together to get a total taxable income, for which the standard income tax is applied, initially ignoring the 10% penalty, and then showing the added 10% penalty on line 59 of the new 2018 Schedule 4 . That gets added to your standard income taxes.....to show a total owed for the year. Then the total withholding from all sources is used as a credit against that total.
So it all works out properly...total withholding vs. grand total taxes and penalties assessed