The state treasurer shall by rule prescribe the time periods for investments in the investment pool and the procedure for withdrawal of funds from the investment pool. The state treasurer shall promulgate such other rules as are deemed necessary for the efficient operation of the investment pool. The rules shall also provide for the administrative expenses of the investment pool, including repayment of the initial administrative costs financed out of the appropriation included in chapter 294, Laws of 1986, to be paid from the pool's earnings and for the interest earnings in excess of the expenses to be credited or paid to participants in the pool. The state treasurer may deduct the amounts necessary to reimburse the treasurer's office for the actual expenses the office incurs and to repay any funds appropriated and expended for the initial administrative costs of the pool. Any credits or payments to the participants shall be calculated and made in a manner which equitably reflects the differing amounts of the participants' respective deposits in the investment pool fund and the differing periods of time for which the amounts were placed in the investment pool.