Have you gone through the current sensor calibration? There are a few ways to do this, but the safest way to get a good estimate is shown in this video: (the beginning of it is just talking, information starts at around 2:25.
Otherwise another way to do this is to measure the value of the resistor next to the actual current sensor resistor on your FC and then you can calculate what the scale/offset should be as discussed in this RCG post
There is also a third way to attach a Watt meter to your quad and measure the current when motors are running and then adjust the current, but you need to be careful when doing this as it can be quite dangerous since you are throttling up your motors with propellers while its resting on your desk!.
