Civ. Code, § 731.04

This chapter shall govern the ascertainment of income and principal and the apportionment of receipts and expenses between tenants and remaindermen in all cases where a principal has been established without the interposition of a trust, except that in the establishment of the principal, provision may be made touching all matters covered by this chapter, and the person establishing the principal may himself direct the manner of ascertainment of income and principal and the apportionment of receipts and expenses or grant discretion to the tenant or other person to do so, and such provision and direction, where not otherwise contrary to law shall control notwithstanding this chapter. The exercise by the tenant or other designated person, of such discretionary power if in good faith and according to his best judgment, shall be conclusive, irrespective of whether it may be in accordance with the determination which the court having jurisdiction would have made.