Thank you.  For IRS, you are correct that 100 - 110% of the 2019 tax has to be spread out evenly to avoid penalty.   Say that the 2019 total fed tax was $10,000 and they paid $2,500 each quarter but ended up owing another $80,000 would they be penalty and interest free?  What if they paid the $10,000 on 1/15/2021 for quarter 4?  Would they be penalized just on the $10,000 that should have been spread out or does the penalty factor in the $80,000?