Unemp. Ins. Code, § 13023

The department may, by regulation, permit employers to estimate the wages which will be paid to any employee in any quarter of the calendar year, to determine the amount to be deducted and withheld upon each payment of wages to such employee during such quarter as if the appropriate average of the wages so estimated constituted the actual wages paid, and to deduct and withhold upon any payment of wages to such employee, during such quarter such amount as may be necessary to adjust the amount actually deducted and withheld upon the wages of such employee during such quarter to the amount that would be required to be deducted and withheld during such quarter if the payroll period of the employee were quarterly.