Your stepper doen't know where it is . You would have to count the steps in one direction and save that in a directioncount variable like Foward_Counts/Reverse_Counts etc. and increment/decrement accordingly. Alternately, you could use a shaft encoder somewhere or add a rubber wheel driven by the turntable and attache the shaft encoder to the rubber wheel. This would require a custom mount for the wheel and encoder. If you add an index opto sensor with a HOME flag like most stepper driven hardware, then you would have a HOME funtion executed on power up that reverses the motor slowly until the flag backs into the opto-sensor slot and then the motor reverses direction and drives "n" counts forward from the flag. This is the Home offset count.
If you then keep track of forward and reverse counts the system (not the stepper) should be able to calculate steps in any direction to any point. This is how most stepper driven systems work unless they have incremental or absolute shaft encoders.