What is generally recommended is that you don't try to calculate any additional interest or penalty. Just go ahead and file your return as you have prepared it. The IRS will calculate any interest or additional penalty and notify you of additional payment due.
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*** I am NOT a tax expert. I am a seasoned TurboTax user, and volunteer to provide assistance to TT users. Nothing I post is to be considered TAX ADVICE; I bear no legal liability for responses.***
