Here is my follow up:
I filed and paid the 2024 California return by February 15, 2025. A while later, I saw that the California Franchise Tax Board (FTB) web site listed that we owed an additional $105, but there was no notice listed. I phoned the FTB fairly late in the day and was told to try again later. I called in the morning on another day and there was a call-back feature. They called me back about 2 hours later. They told me that a notice had just been mailed out that day. They said that the $105 was for underpayment of estimated tax, because I had underpaid in the first two payments. The payroll withholding for the year was less than I expected, so I had made up for it with an extra-large estimated tax payment in January 2025. The total amount of estimated tax payments and withholding were enough for the Safe Harbor Rule, but I had underpaid in the first two payments, so there was an underpayment penalty. I had until April 15, 2025 to pay the penalty before more interest would accrue, so when the notice arrived in the mail, I paid the penalty on ftb.ca.gov.
The $105 penalty calculated by the State for paying in February agrees with what TurboTax said, so apparently TurboTax was picking up the withholding and estimated tax payments, after all. The $600 penalty that TurboTax calculated if I had filed and paid the taxes owed on April 15 sounds too high to me.
