A ballpark estimate? Uncle Sam wants $3-3.5k more and payments through the employers came up a few hundred short of the 90% requirement primarily due to some bonuses in the last quarter.

https://www.investopedia.com/t...RS%20sets%20annually.

Typically, underpayment penalties are 5% of the underpaid amount, and they’re capped at 25%.
Underpaid taxes also accrue interest at a rate that the IRS sets annually.

Also nondeductible.

I have my accountant take a quick look & give me an estimate where we are.
If it's close I send a little extra on my 4th quarterly estimated taxes, due in January.
If I end up overpaid, I credit it to next years estimated taxes. If overpaid a lot I can get a refund.
We're both self employed with variable incomes.