For the purposes of applying AS 13.38.440 - 13.38.490, the value of the trust is the fair market value of the cash and other assets held by the trustee with respect to the trust, whether these assets would be considered income or principal under the other provisions of this chapter, determined at least annually. In the discretion of the trustee, the value of the trust may be averaged over a period of three or more preceding years, except that, if the trust has been in existence less than three years and the trustee decides to average the value, the average shall be determined over the period during which the trust has been in existence.