Cal. Water Code § 51420

At least 90 days before the interest date of bonds or refunding bonds, the county treasurer shall estimate the amount necessary to pay interest and principal maturing on the interest date, crediting thereon the funds in the treasury applicable to the payment thereof, and excluding therefrom any funds deposited pursuant to Chapter 5, Article 4 of this part, or derived from the sale of lands by the county treasurer as trustee of the district under the provisions of Articles 1 and 2 of Chapter 5 of this part. The estimate shall include expenses of the county treasurer plus 15 percent of the aggregate sum to cover possible delinquencies.