California Code, Insurance Code - INS § 10074

The amount of expense so fixed shall not exceed two per cent of the aggregate premiums returned as received, as provided in section 10076.  The whole of such amount, or so much thereof as is necessary, may be assessed upon all insurers or insurance agents who assume risks or accept premiums for fire insurance in said municipality.  Such assessment shall be made in proportion to the several amounts of premiums returned as received by each in the statement required by section 10076.