Try it this way -
If your stepper motors require 200 pulses per revolution and your have the driver card set to 8 pulses per step, then Mach3 must put out 1,600 pulses per revolution of the motor. If you have say a five to one step down on your stepper motor to axis drive, then Mach3 has to put out 8,000 pulses to drive the axis screw round once, and if your screw moves the axis 1/10 inch per rev, then your need to put out 80,000 pulses to move your axis one inch.
In the stepper motor settings if you are trying to make the axis move at too fast a rate, the required pulse rate will exceed the rate at which Mach3 can output pulses to drive them - so you need to move up a stage from 25 kHz. Mach 3 can be set at different speeds, but it is advisable to use the lowest speed for your requirements.