What is the diameter of your turntable? If we talk about a ( small ) 23m turntable, this are ~26cm in H0 gauge. This means a circumference of 83cm. To position the rails exactly, you need a positional accuracy of less than 1mm - say 0.5mm , what means 1660 steps per rev!!.
Your belt drive has a reduction ratio of 3:1 and your stepper has 200 steps/rev what means 600 steps/rev. That is far too few. When using the current driver suggested by @jremington you can use microstepping, but the microstep positions aren't that exact as the full step positions. I would suggest a much greater mechanical reduction ratio.