I filed my 2017 tax return in October 2018 and had paid all the tax due (as I calculated at the time) prior to the payment deadline of April 15, 2018. Recently, I discovered an error in my tax calculation -- I had not realized that I need to realize Bitcoin Cash acquired through a Bitcoin hard fork as taxable income -- so I realize I underpaid taxes by a little over $2,300. I need to file Form 1040-X for this. Although I could just file it and then leave the calculation of penalty and interest to the IRS, I would like to do an approximate calculation and send the total money via EFTPS so that if there's a delay in the communication from IRS, I don't end up paying extra. I expect to file the 1040-X some time in January 2021.
From what I can make out based on the instructions for Form 1040-X, failure-to-file penalty probably doesn't apply? And I am quite unsure about failure-to-pay penalty. But I cannot find anything explicitly saying or rejecting the idea. Notice 746 that describes penalties and interest does not seem to even mention the 1040-X.
It seems clear that I need to pay interest; I'm just not sure about the failure-to-file and failure-to-pay part.