Because of the Covid-19 changes I know the calculation of all interest rate penalties have changed and the IRS has issued rules in this regard, but I haven't followed along. The 5% rate is correct for the first 2 quarters but changed to 3% for the third quarter. The interest is calculated on a daily basis and compounds daily, and it's based on the cumulative unpaid estimated taxes.
It's not going to be a hell of a lot of money in any case, and depending on your situation the $40K estimate might not really be the right number to use because your 10/15 payment might get you into a "safe harbor", (or not). Let the IRS calculate it for you.
