Axis scaling?

I purchased a 3040 cnc mill/engraver on ebay 9 years ago for my kids (and me) to play with.  It has been sitting up for a while.  I have installed linuxcnc on a different computer and I am going through the stepconf.  At one time this thing cut pretty accurately.  I am having trouble getting the axis scaling correct.  The x-axis motor drives a ball screw for which I have measured the pitch to be 4mm (I turned it manually 10x and it moved 40mm).  The motor has 1.8deg stamped on it.   

On the 'Axis X' stepconf screen I have the following settings:
Motor steps per revolution: 200
Driver Microstepping: 8
Pulley teeth (Motor:leadscrew): 1 : 1
Leadscrew Pitch: 4 mm/rev
Maximum Velocity: 10 mm/s
Maximum Accel: 10 mm/s^2

When I run 'Test this Axis' and set the distance to +/- 100mm it actually moves back and forth from 0mm to ~88mm.  It does seem to be losing any steps.  What am I missing?

BTW - I Tried 1, 2, 4 and 8 for the microstepping.  8 is the closest.  

thanks in advance

I would also suggest looking at the stepper timing. A machine from ebay may not have stepper controllers that can handle short step pulses.
Search the forum for "stepper timing" or "StepLen" in your INI or HAL file.

The values that control that are:
StepLen, StepSpace, DirHold, & DirSetup

If you do not intend to move too fast, you can set all of these > 10,000 without a problem. For testing you can set them safely to 25,000 or higher for (DirHold and DirSetup you can go even higher). See if that helps then tune if back down after.

If you can find the datasheet for the type of steppers you have then that should have the proper values for your drives.