Have you looked for tutorials on reading mouse encoders?
A mouse is not an exact input device.
It works on a computer because you use it to move the pointer until the pointer gets to where you want it.
Without the screen, it is not very easy to use it to enter exact values.
As you move it in any particular direction, small changes in how you hold it turn the mouse relative to the movement direction, translating some of the movement into input along the perpendicular axis.
however you DO decide to define the desired navigation coordinates, how do you intend to guide your BB8?
Again, guiding it by remote control uses YOU as the navigation system. If you want to make it autonomous, you will not have the correction factor in place that you previously provided.
Want to experiment? Plan a route. Up for 1 second. Left for half a second. Down for 1 second. Right for half a second.
Close your eyes and control your BB8.
Did it end up where you started? Repeatedly?
If your arduino is going to tell the BB8 to move in a particular way without and location awareness, you will not be happy with your results.