Can you accurately calibrate your current without an external power analyzer?
The default value is fairly accurate. But you will need something to calibrate it against for better accuracy. And you are only as accurate as that something i.e. power meter or ammeter
Better accuracy requires a better sensor circuit.
Is the default value 17 or 18?
In APM Planner 2.0 you just pick the option for you setup, so it depends on autopilot and sensor or power module used