When you get the kit installed and you are ready for the steppes per.... Take the steps per rev of the motor DIVIDED by your step increment on your driver boards such as 1/2, 1/8, 1/16, this now gives you how many steps the computer must put out to rotate the motor 360 deg. now that you have that you need to multiply that by the screw pitch.
Here is an example for a 200 step per rev motor, on 1/2 stepping with a screw pitch of 10TPI. (200/.5)6 =2400 this means it will take 2400 steps to move your machine 1 inch if you have those parameters. BUT THIS IS NOT ACCURATE!!!
In mach 3 there is an axis calibration tool that once you get that rough estimate you will use a dial indicator to measure how far the bed moves when you tell it in mach to move a specified distance. It then asks how far it actually moved and adjusts your steps for you.
Here is a video Hoss made that shows what I mean
[nomedia="http://www.youtube.com/watch?v=XkO5tc-jSxw"]YouTube - Improved Mach 3 Axis Calibration[/nomedia]