One thing to keep in mind is that if you're on QF metal from JFK to LAX, you will only earn J SC's as they replaced the 747 on QF107/108 with an A330, with only J and Y cabins.
As mentioned, it's SC's based on your membership year, although you can easily earn SC's on AA with YUPP/KUPP fares (which could easily push you to Gold (OW Sapphire) @ 700Sc's).
I also have my membership year ending end of Feb each year. I re-qualified for Qantas Platinum (OW Emerald) in October last year, when I had status thru 28 Feb 2011. I now have Platinum thru Feb 2012, even though this membership year has another 19 days to run:
(SC's removed by me).
If you do get to Gold, and you do not re-qualify, QF will only 'soft' land you to Silver if you have met the minimum 4 segment requirement on QF. If you haven't, you'll fall straight back to Bronze.