Before the 15th day of June of each year the board shall estimate and determine the amount of money required by the district for purposes of the district during the ensuing fiscal year and shall apportion this amount to the counties included within the district, one-half according to the relative value of all the property in each county within the district as determined by the board and one-half in the proportion that the population of each county bears to the total population of the district. For the purposes of this section the board shall base its determination of the population of the several counties on the latest official census information available to it. The total amount of money required by the district for district purposes during any one fiscal year shall not exceed one-half cent ($0.005) on each one hundred dollars ($100) of the assessed valuation of all the property included in the district.