If you really want to know your battery capacity, get a digital volt meter and make a plug that you can plug into your reciever. Connect the red and black wires to that plug and turn on the reciever. Now while on the ground, act like you're flying the plane. Monitor and plot the voltage every minute (this is really easy if you can plug your DVM into a computer). Look for the rapid drop off in voltage (will typically happen around 4.65-4.7 volts). Use that plot of time vs. voltage. If you typically fly for 10 -15 minutes, back up from the rapid drop off time 20-25 minutes (safety margin). That voltage should be your no go voltage.

Brad