well what you need to do is to divide the value of a pot into half
so i would say that 507 until 517 is a good middle value.
then take the value between 517 until 1023 and map it with the min and max of the stepper speed.
after that take between 0 to 507 value of pot n map it with the min n max of the stepper speed.
third is that determine which side of the pot make the stepper move clockwise. what i give here should give you an idea on how to do it