Re: transformer impedance
It is not so much ?calculated,? as it is a design choice made by the manufacturer. It is also a design decision made by the engineer or contractor, to choose the standard value offered by the manufacturer, or to special-order a different value. It is expressed as a percentage, based on the rated KVA and the rated voltage of the transformer. The formula S = V*2/R can be used to correlate the three numbers.
A low value of impedance will give you lower energy loses internal to the transformer, and will require a lower value of air conditioning to cool the room in which it is installed. It also gives you a lower value of voltage drop to downstream components, as it gets loaded higher and higher towards its rated power.
A high value of impedance will limit the fault current available at the secondary terminals of the transformer, and therefore to all components downstream. This is sometimes needed, if the switchboards or panels selected for the job have a small value for their rated fault current.
That should give you a start. But is there something specific you were looking for?