Check the manual on the triplite for the cut off , I have tested it with some other bateries, and its usualy around 10.5 - 11.00 Volts. I know my car will crank with 10.0 left in the batery.
A good service center / batery shop will have a load generator. find out your target voltage, then use the load generaor ( along with your headlights, and AC to run the car down to the target voltage, then try and start it. if it wont start, run back inside and tell the guy you need a jump