Ok, than I missunderstood completely. I thought you want to define the angle with your joystick where the stepper should be positioned to.
From the datasheet of your stepper I see that is has a 50:1 gearbox. Theat means 1 revolution is not 200 steps, but 10000 steps.
What speed do you want to achieve? With a gearbox it will be rather slow.
What about the compass. This will than define the angle to be positioned and not the speed? How should this work together with the joystick?