It depends.
You indicated that you received a significant income spike or windfall in December.
The US tax system is a pay-as-go system. That means you do not have a tax liability until income has been received.
Your observation he sooner you indicated you were paying the balance due the less the penalty was also indicates there was less penalty because the time between when the income was received and when the tax was being paid was shortened (as opposed to waiting until April 15th).
When you are able to access the TurboTax Amend return feature on the 25th, you can check the actual Form 2210 calculations based on your individual circumstances.
You may not have been subject to any late payment penalties at all and can amend to recover any excess tax you paid when you filed.
