You're right, this is an important aspect to have into account. The software calculations depends on it.
In this picture you can see the front of the PXFmini:
The front is the PWM rail.
About the ESC calibration, I would suggest you calibrating the RC first, in case you haven't. My colleague @LanderU says that some times, those ESCs requiers to calibrate more than once because they fail. I guess you have try it out more than once.
In order to be sure that the RC is working fine, open a GCS and check if the stick movements are well mapped: check if you increase the the left stick in vertical -> throttle increases. The same with pitch, roll and yaw.
You could use APM:Planner RC calibration tab to see the RC values:
Here you have a calibration guide under APM:Planner: