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.
example:
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
Simon