For section 11A, 11B or 11C, the amount of ordinary remuneration that, but for the cessation of employment by a person, would have been payable by the employer to the person for a period of service referred to in that section shall be calculated—
(a) at the rate at which ordinary remuneration was payable to the person immediately before the cessation of employment; and
(b) if the person was a part-time employee or casual employee immediately before the cessation of employment—on the basis that during that period of service the person would have been employed for the number of hours per week that is equal to the average number of hours per week for which the employee was employed during the period of 12 months immediately before the cessation of employment.