California Code, Revenue and Taxation Code - RTC § 19026

If, after paying any installment of estimated tax required by subdivision (b) of Section 19025, the taxpayer makes a new estimate, the amount of each remaining installment (if any) shall be the amount which would have been payable if the new estimate had been made when the first estimate for the taxable year was made, increased or decreased (as the case may be) by the amount computed by dividing--