I filed my 2017 United States federal income tax return in October 2018, after paying all due taxes by April 2018. The IRS found a calculation error and sent back an extra refund in November 2018 along with a Form CP-12. In December 2019 / January 2020, I found another error that led me to file a 1040X. When filling the form, I used for my column A (Original Amount) the values as determined after applying the CP-12. The first 1040X was successfully processed by IRS and they accepted my payment.
Yesterday, I realized that I need to make another amendment, due to which I have more tax due. When filling the Form 1040X column A (Original Amount) should I use the amount as it would have been after applying the first 1040X (i.e., the column C of my previous 1040X becomes the column A of my new 1040X), or should I use the use the amount as it was prior to my first 1040X (i.e., the column A is the same across both)?
Also, relatedly, how do I fill in rows 17 and 18? Do I fill in the same values as I did for the original 1040X, or do I add the amounts paid up with the 1040X?
The final outcome in terms of tax due doesn't change, but I want to fill it in a way that is smoothest for the IRS to process.