163(2)
Where, for the purposes of any of the provisions of this Part of this Act, value falls to be calculated by reference to the duration of a tenancy, and, by reason of any option or other contractual right with respect to the determination, renewal or continuance of the tenancy, the date of expiry of the tenancy is not ascertainable with certainty, that date shall be taken to be such as appears reasonable and probable having regard to the interests of the party by whom the option is exercisable, or in whose favour the right operates, and to any other material considerations subsisting at the time when the calculation of value falls to be made.