Your not going to be able to control the current that precisely with just any pot. You would need a 1 Megaohm pot and even that would not work that well. The whole idea of using a pot driven by a stepper motor for this project is wrong. First of all , you need
a regulated 28V supply . If you get three 12V batteries to make 36V you can get a buck dc to dc converter that you can adjust to 28V and you'll have your regulated supply. Then you need to switch resistors to get your different currents.