Cal. Rev. & Tax. Code § 7659.1
Upon written notification by the board, any person whose estimated tax liability under this part averages nine hundred thousand dollars ($900,000) or more per month, as determined by the board, shall, without regard to the tax in any one month, make a prepayment as prescribed in this section. The prepayment requirement may be satisfied by making a prepayment of an amount not less than 95 percent of the tax liability for the month to which the prepayment applies or a prepayment not less than 95 percent of the amount of the tax liability reported for the previous month. Prepayments shall be made for the monthly periods designated by the board and during each succeeding monthly period until further notification by the board in writing.
Ca. Rev. and Tax. Code § 7659.1