I believe Sherline's motors are standard steppers that have 200 steps per revolution. Their rotary table has a 72-1 gear ratio. Make sure the A axis is defined as rotary, with the units in degrees and not mm or inches. The idea is to define one degree in terms of the steps (or microsteps) your drivers are set to. So the math would be 200 times 72 divided by 360 for the number of whole steps per degree (40). Multiplying that by your microstepping factor should give you the correct value to put in the box in the Motor Tuning dialog.