Did the client have California withholding 2019?  Did the withholding in 2019 exceed their tax liability in 2019 or was the tax due after withholding less than $500?   If yes, that is an exception to the underestimated tax penalty calculation for California.  And if they meet that condition, they could have $25M in income in 2020 and no withholding and there would not be a penalty.