With the above information,

200 full steps (1 revolution) will move the axis 0.2". So each step moves the axis 0.2/200 =0.001"

So you need 1000 steps to move the axis 1 inch. Now, if you have a 10 micro step drive then you need ten times the number of steps to move the axis an inch, so the steps per unit would now be 10,000.

If your stepper drive is set to 8 micro steps then it is 8000 (8 x 1000) steps per unit. By the way, there is no advantage in settings the drive micro steps to anything greater than 10.

The above assumes that you have set the native units in Mach3 to inches.

Cheers,

Peter