With the PM RRE, they normally set it to start at the normal start time of PM ready (in this case 12:30) so you report to the airport at 12:30 then that starts both your :45 prep time (so the earliest departure you should be scheduled for would be 13:15) and your 6 hours of ready reserve also starts at 12:30 so you would be released at 18:30 and any departures assigned would need to be scheduled to leave before then.