Worth pointing out that the CAA rules affect British registered aircraft, not Australian.
The minimum crew is set by a combination of the 1/50 or part rule and a requirement for a crew member at each 'Main' door. Hence, for example, a 747 with 10 main deck doors and 2 upper deck doors would require a minimum crew of 12, one for each main door, even though the actual passenger capacity is not between 551 and 600 (1/50 or part). There is some scope for minor variation, but this must be authorised by the CAA: For example a charter 757 (a narrow body aircraft) in a 235 config has a legal minimum crew of 5 (1/50) although it has 8 'Main' doors. The minimum rises to 6 on ETOPS sectors, because of the life-raft requirement. A 737 has 4 'Main' doors with a minimum crew of 3 (737-800 can be 4) under the 1/50 rule.
737 o/wing exits are not classified as 'doors', but 'exits', and in the UK, classified as 'Self-Help' exits i.e. a briefing to pax is sufficient, together with restrictions on who may occupy those seats.
Life rafts are carried as additional equipment on ETOPS sectors: PER-SYD may fly over water, but probably not under ETOPS flight rules and therefore there would be no requirement to brief for ETOPS equipment.
The long and the short of it is that the minimum crew is set by the licensing authority, and airlines do not have the right to vary the minimum on an ad-hoc basis.
May we now move off this subject and back on-topic please