If the calculation and payment of the commission cannot be completed until sometime after the regular pay day for the workweek, the employer may disregard the commission in computing the regular hourly rate until the amount of commission can be ascertained. Until that is done he may pay compensation for overtime at a rate not less than one and one-half times the hourly rate paid the employee, exclusive of the commission. When the commission can be computed and paid, additional overtime compensation due by reason of the inclusion of the commission in the employee's regular rate must also be paid. To compute this additional overtime compensation, it is necessary, as a general rule, that the commission be apportioned back over the workweeks of the period during which it was earned. The employee must then receive additional overtime compensation for each week during the period in which he worked in excess of the applicable maximum hours standard. The additional compensation for that workweek must be not less than one-half of the increase in the hourly rate of pay attributable to the commission for that week multipled by the number of hours worked in excess of the applicable maximum hours standard in that workweek.