Your tax is only calculated on your 2020 tax return, no matter what you might pay as estimated taxes.
A conversion from a SIMPLE IRA to a Roth IRA should be reported on form 8606 on lines 16-18. The taxable amount of the conversion is transferred to line 4b of your form 1040 and added to your other income for the year. Your total income tax is on form 1040 line 24.
Then, any tax that was withheld on the conversion will be reported on line 25b of your form 1040. It will be added to any other payments or withholding, plus any credits, which equals your total payments on line 32.
If your payments are more than your tax, you get a refund. The Roth conversion increases your income and increases the tax you owe, but you still might get a refund if your total payments are more than your total tax.
