The taxable amount is the full amount of 24,000. The whole amount is taxable. Then you get credit for the withholding . But yes if your taxable income is reduced to zero you will get the withholding back. But you might owe the 10% Early Withdrawal Penalty on the amount you didn’t convert to the ROTH. Which would be the 12,000 held out for taxes. So you need to replace the withholding from your own money when you put it into the ROTH.
