You have a 200 step per rev motor. 1.8 degrees. I assume that is right.
Multiply that by whatever "micro steps" your stepper driver is set at. Lets assume ten micro steps for example.
200 x 10 = 2000 steps per motor revolution.
A 10 to 1 gearbox reduction. so 2000 x 10 = 20,000 steps per axis revolution.
Divide that by 360 to give steps per degree. 20,000 / 360 = 55.55555 Enter this number in Mach3 "steps per unit" for A axis.
Again, this is assuming some things. Use the appropriate numbers in the above calculations and all will be sweet.