From the Castle Drivers Ed little book thing, pg. 36.

Problem: My Castle ESC may or may not arm, but it will not calibrate to my transmitter.
Solution: Most calibration issues can be solved by changing settings on the transmitter. Make sure you have both your throttle and brake endpoints (called EPA or ATV on your radio) on the throttle channel out to between 100 to 120%. Make sure if you have a Futaba or Futaba made transmitter to have the throttle channel set to the reversed position.