Another way of looking at this, and the way it's measured during factory acceptance testing is:
Short circuit is placed on the LV connections, then simply apply a voltage to the HV connections to create Full Load current in the LV winding.
The percentage of HV voltage required to do this is also the percentage impedance.
600V required on HV to circulate Full Load current in the LV.
If HV rated Voltage is 12000V
The %Z is 5% or 600/12000
Hope this helps