2. The department shall estimate annually in advance the amount necessary for the administration of this subsection and the maintenance of this fund pursuant to this paragraph.
By July 1 of each year, the department shall calculate the assessment rate, which shall be based upon the net premiums written by carriers and self-insurers, the amount of premiums calculated by the department for self-insured employers, the sum of the anticipated disbursements and expenses of the Special Disability Trust Fund for the next calendar year, and the expected fund balance for the next calendar year. Such assessment rate shall take effect January 1 of the next calendar year. Such amount shall be prorated among the insurance companies writing workers' compensation insurance in the state, the self-insurers, and the self-insured employers.