(a) The board of trustees shall determine the principal and income of the retirement fund, workers' compensation fund, employee benefit fund, and other funds on periodic evaluation dates established by the board of trustees, but at least once every 12 months. Income shall be determined by the board of trustees in accordance with an established method which it may prescribe in its bylaws and which conforms to acceptable accounting practices in use by corporate trustees. Income shall be added to and become part of the principal of the various funds on the periodic valuation dates established under this Code section. The board of trustees shall deduct from either principal or income, or both, such charges, expenses, and liabilities as it determines in its discretion to be chargeable against either.

(b) The principal value of each unit into which the funds are divided shall be determined on each such periodic valuation date by dividing the then principal value of the funds by the number of units into which it is then divided.