my understanding of mach 3 is that you will have to set it up so that it knows how many steps per inch.. on each axis... it takes care of all the rest of the math.. you will have to spend some time tuning it for the motors as well, a heavier load will take a longer accel and decel time....
so just a short example, if you have a 200 step per rev motor, and go through 1:2 gearing so that the motor now turns 400 steps per rev of the screw, and your using a 10 TPI screw, then thats 4000 steps per inch.. thats what you'd put into mach3...
Grizzly X3, CNC Fusion Ballscrew kit, 3 500oz-in bipolar steppers, 3 203v Gecko's, Linear power supply from Hubbard CNC, Mach 3, BOBcad Pro Art V22, Rhino.