Needless to say, this wasn't how the rules were intended to be used.
I think you will find that's exactly how the rules were supposed to be used. It's quite common, particularly in long haul, for the cabin crew to report earlier than the flight crew. Using the flight deck report time if the CC's report time fall into a more limiting band allows the whole crew to run to the same finish time. If the CC's report time is not in a more limiting band (more usual) then the extra hour FDP that CC are permitted to work allows the same thing to happen. As the previous poster states though it is very clear that your FDP starts when you report it's just the band that limits it that is based on the flight crew report time. For rest purposes it's your actual FDP start time to the off-duty time that decides how long you are due, but again you are due one hour less than the flight crew in all cases (including 12+ hours duty) so the rest period should work out the same as the flight crew.
Sounds like a bit of con?, well I guess it is but thats the rules as they stand at the moment.