The old Arducopter battery MAH remaining failsafe was 520mah. That was 10-12% battery remaining, and about 1.4 minutes of flying time. In the latest solo Arducopter firmware (1.5.2), it was increased to 670mah. That will kick the failsafe in at 12-15%, and be just shy of 2 minutes flying time. This is probably not a bad idea. You just need to expect it as part of your flight planning now. If it is impractical for what you're doing, you can change it back to 520, or whatever you want.
FS_CURRENT_RTL is the new parameter it uses in the distance based battery failsafe, and the default is 30. I presume this is amps. And I presume this is telling the calculation how much current is used in flight. It is really only using 20-22 amps though, not 30. Which explains why the failsafe is kicking in so damn early now. I'm getting failsafes kicking in around 20% thanks to this, which seems ridiculous.
So reading about it in the github notes, I found "helps ensure that energy-based failsafe results in at least 10% remaining on landing." So basically they artificially increased the calculated current use to force the failsafe to engage sooner. I changed it to 22 amps to be more realistic. But I understand why they did it, since they're coding it for the lowest common denominator of users.