Calculating current draw from a battery using voltage and time information
Measure the current directly. Put a low ohms value resistor in series with the load and then measure the voltage drop across this resistor. Trying to do this by the schemes you are trying are at best an experiment. 

You are in selection mode not high volume production mode. A few extra components and a little extra circuitry to do it the direct and correct way is the best. The cost is inconsequential.