No sum in excess of 5 percent of the net income derived from an endowment care fund, or special care fund, or both, in any year shall be paid as compensation to the board of trustees for its services as trustee. This amount shall be the total compensation from the fund to be paid to a trustee for services. For purposes of this section, “net income” means the amount of income remaining after reasonable administrative expenses, including bookkeeping, postage, taxes, and other costs directly related to generating income to the trust fund, have been deducted from the gross income derived from the fund.