Adjustable power supply from +5V to -5V
You may want to consider a unipolar to bipolar level shifting circuit that is based on a power op amp. An example schematic is shown below. Instead of the DAC, you could simply use a potentiometer.
Regarding calculations, there is a good application note here, which should help you hit the ground running.
Keep in mind that you will need to ensure that the op amp can source/sink the necessary load current.