55 This is best illustrated by considering the consequences of determining the non-parole period by reference to a fixed proportion of the head sentence. First, the non-parole period will be the same for all head sentences of the same length. Second, the length of the non-parole period will increase as the head sentence increases. The use of a fixed proportion allows for wide numerical variations in the length of the non-parole period to reflect wide variations in the length of head sentences. That is, the use of the same fixed proportion to different head sentences does not produce numerical equivalence. Third, the proportion between different head sentences imposed on different offenders and associated non-parole periods (and parole periods) will remain constant. For example, if the proportion of the non-parole period to the head sentence was 66% for both Mr Lam and Ms Trinh, Mr Lam's head sentence, non-parole period and parole period would all be 1.4 times longer than that of Ms Trinh. That outcome can be seen as consistent with the rationale for parole. [In fact, Mr Lam's head sentence is 1.4 times longer than Ms Trinh's, his non-parole period 1.8 times longer and his parole period 8 months shorter]. Fourth, the length of the non-parole period is informed, indirectly, by all the relevant sentencing considerations and principles that inform the determination of the head sentence. For example, positive factors for or against the prospects of rehabilitation will be reflected in the length of the head sentence, even when general deterrence is a weighty sentencing consideration.