Iâm not sure the 20% substantial understatement penalty applies in the context of somebody whose only goof was that the person did not pay their quarterlies. As I recall, the substantial understatement penalty applies only when a return has been filed by the taxpayer, and the amount of tax shown on that return is less than the amount that SHOULD have been shown by 10% or $5k, whichever is greater.
I donât have the time to go back and research this now, but in going from memory, I donât think the 20% substantial understatement penalty comes into play if the taxpayer files an accurate, timely return but failed to pay the quarterlies along the way.