If your goal is avoiding damaging the new batteries, the best advice is to monitor them while driving, rather than estimate a range and "hope" it's correct.
You want to avoid drawing a 12v lead acid battery below 10.5 volts under load -- a.k.a. the battricide zone.
You should get a Pak Trackr or similar.
I use a DIY LED bar graph to keep an eye on (avoid murdering) my small 48v pack:
http://ecomodder.com/forum/showthrea...raph-8216.html