Refering to a battery pack as a Polymer pack doesn't really say what kind of pack you have. So it's dificult to give you a correct answer to your question. The most common are lipo. Lipo cells can be monitored the same way common flashlight batteries or lead car batteries are monitored. As indicated by chil6er you can simply monitor the voltage. However, if you have lifepo4 I wouldn't recommend this method. Lifepo4 cells tend to hold a more linear voltage until they are close to death. Therefore, the proper way to monitor lifepo4 cells is by the amount used not the amount left. To do this you need an amphour meter. You need to assume that the pack is full (12ah in your case) and the meter will record what you discharge. So if your goal is to discharge 10AHs you would just watch the AH meter till you discharge 10ah. This leaves you with 2ah remaining in the pack.

However, no matter what type of pack you have the lower you discharge it the shorter it's life. Also the faster you discharge it the shorter it's life. A good general rule is 50% discharge will increase the life of the pack 25 to 50% over an 80% to 90% discharge. Likewise, this is simular to the discharging at rate. Discharging at 2c will shorten the life by 50% over a 1c discharge rate.

Now lets get really deep. You should never trust any monitoring methods 100% at the "pack level". Monitoring the pack voltage for example only means the "pack" is still happy. But it does not mean the individual cells are happy. Therefore, you should always have an individual cell monitoring while discharging the pack. Some BMS monitor at both cell and pack levels but not all or even many. And you should never assume that all your cells are being properly charged to a full charge level.

You need to give us more info on your pack. A pic of it and your charger would be nice.

Bob