Those with a Dec use year may bank up to 100% of their points until May 31. You can bank up to 50% of the total points by Aug 31 and up to 25% by Sep 30.
The percentages are figured on the total number in your contract- not the remianing points- so if you have 200 total and have banked 100 or more bu May 31- you can't bank any more points for that use year- since you have already banked 50% of the total.
If you later find you'll need points for a trip during the current use year- and have already banked your points- you can still borrow from the following year, just as golfnut suggests.
Enjoy!