What you're looking for is called "state of charge". http://en.wikipedia.org/wiki/State_of_charge
The problem with looking at open circuit voltage is that it varies with respect to temperature. So you would need different cal curves if you application has a wide temperature range, and you would need to perform a temperature measurement.
It may also be misleading because it will sometimes rebound after a short period of time after the battery has been loaded. (depending on chemistry, temperature, etc). So unloading the battery and then checking to see the OC voltage could give you lower voltage than if you were to come back 10-minutes later and check again.
http://batteryuniversity.com/learn/article/how_to_measure_state_of_charge
Actually measuring how much current has flowed out of the battery is a more accurate approach. It's called coulomb counting: http://www.linear.com/product/LTC4150