Gyro bias gets calibrated with each battery plugged in. Quad must be perfectly motionless. It is best to put quad on ground, plug it in while moving it as little as possible, and wait for LEDs to say it is done (fast LED flashing stops).
You will not get the best bias calibration if you are holding it. Not humanly possible.
16.09 has a feature to wait until motionless before doing bias calibration, but it can be turned off, and it did not exist before 16.09.