Calibrate is not the correct term, rather one needs to characterize one's specific servos being used as all don't have the same total travel range and the same pulse width range to reach those end stops.
Using servo.writeMicroseconds(uS) one can with some time invested determine the exact end stops and total travel range. The arduino library of degrees of movement using the servo.write(angle) command is an abstraction that makes assumptions for servos in general rather then any particular servo.