Calibrating the 0A offset removes most of the error from the current readings. If you require the full accuracy that the current sensor offers, you will also need to calibrate the BATT_AMP_PERVOLT parameter. I don’t recommend doing this unless you are familiar with high-power electronics. For most users, a value of 50 is good enough.
You will need an accurate external load of some kind, such as a programmable artificial load or precision power resistors. Attach the load to the ESC power pads and back-calculate the multiplier based on the equation of a line. Set the OFFSET and PERVOLT parameters to 0 and 1 (same as step #2 in previous post) so you can read the direct analog voltage in Mission Planner.
Representing the equation of a line as y = mx+b:
y = the current in Amps (this should be known based on your external load)
m = BATT_AMP_PERVOLT
x = analog voltage from sensor
b = BATT_AMP_OFFSET
You can solve for m, as you will know all of the other values. I recommend you choose at least 2 current values and take the average result.