If you are worried about how low you can go with RX batteries, then you have not read many of the threads within this forum.

Get yourself an Extended Scale Voltmeter and it will tell you when to stop flying. You CANNOT use a standard voltmeter for that, you must use one that places a load on the RX battery (and the TX battery for that matter) and gives you a good indication of when it's ok to fly and when you should recharge.

http://www2.towerhobbies.com/cgi-bin...&I=LXWW16&P=ML

But based on your question, it sounds like you are trying to 'push the envelope' and fly on a marginal battery. If so, please make sure your home owners liability insurance is paid up, you are a current AMA member, and that you have a plastic bag with you to collect the remains of your plane after you finish with your suicide flight.