You can't reach the full range of the ADC converter in the microcontroller with a Hall sensor, unless you use additional circuitry or have a way to set a different reference voltage on the ADC. Even in optimal situations a common hall sensor will swing the output voltage only between 0.5 and 4.5 volts (approximately) and this means that 20% of the full range of the converter won't be used. This is generally not a problem, since you would find it really hard to position a control with the accuracy of the remaining range, and indeed many joysticks only have 256 "steps" per axis. You just have to calibrate the controller to make use of the available range, something that the firmware can probably do automatically by assuming the controls to be centred when it is powered up, and extending the maximum and minimum value for each axis every time a new value exceeding the current one is reached.