Cal. Health & Saf. Code § 40371
Before the 15th day of June of each year, the regional district board shall estimate and determine the amount of money required by the regional district for its purposes during the ensuing fiscal year and shall apportion this amount to the counties included within the regional district, one-half according to the relative value of the real property of each county, or that portion thereof, within the regional district as determined by the regional district board and one-half in the proportion that the population of each county, or that portion thereof, within the regional district bears to the total population of the regional district.
For the purposes of this section, the regional district board shall base its determination of the population on the latest official information available to it.
Ca. Health and Saf. Code § 40371