"401K from the previous company. "
Generally, once you already have funds in an IRA, you can't do a tax-free backdoor Roth contribution.
You may convert any dollar amount you want from IRA to Roth IRA.
See mandatory Form 8606 to find out the effect of any non-deductible contributions (called basis) made prior to your conversion on the resulting taxable amount.
The actual tax will depend on your tax bracket.
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