As Far as I am concerned the ED meter is the best way to go with A123's. It breaks down each individual cell under load and lets you know A: if the pack is balanced, B: the voltage of each cell. E/D recommends no less then 3.30 per cell. They claim that at that voltage there is still 25% left. It will give you the voltage to the 100's column. Example: 3.35. Extremely accurate.
Really easy to do. You can monitor your cells after each flight in a matter of seconds. There are several guy's at our field using them and everyone of us swear by them. They will also balance the cells for you if you wish.

http://www.electrodynam.com/rc/EDR-207/index.shtml

Stan