If everything is working normal, you should be able to use the biggest part of your batteries. I have put a bit over 2,000 mAh back into a 2100 mAh and the voltage checked out fine. It was still well into the safe range. I think it is better to not use that much, especially if you are depending on it for your servos and Rx to get you home. Just be sure to watch for your motor suddenly slowing down. When that happens, you should immediately land the plane. The idea is to give you a little reserve, not to allow you to continue flying for fun. There are usually two settings for the cutoff. One is to just slow down, the other does a sudden motor cutoff! Not good for a heli! I would use the slow down method and watch for it. There should be instructions for setting it.