Cal. Wat. Code § 12878.9
If a maintenance area is divided into zones, the zones shall be given a numerical designation starting with Zone 1 which shall be the zone which receives the greatest proportional benefit. This benefit shall be expressed as 100 percent and the benefits received by the other zones shall be expressed in terms of relatively smaller percentages as such benefits compare with those received by Zone 1. The map of the proposed boundaries of the maintenance area shall also show the boundaries of the zones, if any, therein and the percentages of benefits to each such zone. The amount to be collected from each zone of a maintenance area shall be determined by the following formula: the assessed valuation of the land within the zone, multiplied by the percentage of benefit for that zone, multiplied by the total amount to be collected in the maintenance area; divided by the sum of the various products obtained by multiplying the assessed valuation of each zone in the maintenance area by its corresponding percentage of benefit.
Ca. Water Code § 12878.9