Determining the amount of estimated taxes

Estimated taxes (plus any tax withholding - I have none) should cover 100% of the prior year’s tax burden to avoid underpayment penalties. For 2019 taxes, this is found on line 12.

Question: is it line 12a or line 12b? Because I had to repay advance premium tax credit, the two lines are quite different. I think it should be based on line 12a, since that is just the income tax, but I want to make sure.

Here are the relevant lines from my 2019 1040:

If I get to use line 12a, I think I don’t need to make estimated payments at all (it is below $1,000). But I will anyway because it is so close to the limit and there’s a good chance I’ll be over for 2020. I really don’t want to have to use line 12b, because I will not have that high a tax burden in 2020.

Am I doing this right?

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