RE: What voltage on Tx is too low to fly safely?
It is not hard to find out for yourself in a manner that guarantees accuracy. Just fully charge your transmitter then note the time and turn on the transmitter. Let it run until the low voltage alarm goes off. At that time turn on a known working receiver and see if the transmitter still operates the system (it will). and note the time and voltage. Now you know how many hours you can safely run your transmitter and have reliable operation. Chances are it will be quite a bit longer than you thought it would be. Now you have actual known data and do not have to take any ones word for it. Now, as your battery ages, the time will decrease but the voltage at the point you should recharge will not, just now monitor the voltage and you will know when to stop flying until you recharge.