How to find out what level of battery voltage corresponds to 50% discharge?
Terminal voltage will only give you a very rough estimate of state-of-charge, since it doesn't take into account capacity fade, temperature, chemical additives, and surface charge, all of which will affect the terminal voltage.  However, if a rough estimate is good enough for you (say, +/- 20%) then the number you're looking for is roughly 12.2V.  



Source: Battery University, which also gives you some alternate methods of measuring state-of-charge.