Microstep management

blockbuffer wrote:

How does linuxcnc decide to call for steps vs microsteps at run time? I can't see how or where the number of microsteps per step (even if '0') is defined.

It does not care about microsteps, that is something stepper drives do, not Linuxcnc, Linuxcnc only needs to know how many steps per unit (mm or inch) to send. That is only needed when using wizards as it makes it easier to just enter values and let the wizard do the calculations.
You can fix or tune the step scale from a working Linuxcnc, it is on the machine menu in "calibration", so you can enter a value, press "test", go to MDI and enter a known value in MM or Inch and measure if it did that exact distance. If not, change value, test again.
Just do not forget to click on "save to file" when you're done tuning.
Or,
Check the drives to what microstepping they are set, it is always written on their side and they have small DIP switches to set that and the current to the motors. Also check what is the reduction ratio between motor pulley and ballscrew or leadscrew and what is the pitch of that ball/lead screw. Most probably the motors are attached to the screws in your case so the ratio is 1:1.
Stepper motor usually need 200 steps per revolution, example screws of 5mm per turn, and by 4 microstepping:
that would be
200X4=800/5
or 160 steps per 1mm of movement.