Transformer output voltage calculations using load current
Rated Voltage at rated current for a source impedance of Z = 6% pu means that the source impedance is 6% of the rated load impedance so for rated current they allow a 6% series drop and short circuit current is 1/6% or 16.7 x rated current and no load voltage is 6% higher thus lower loads will be proportionally less 

or given %Ipu, %Zpu , what is %Vpu?

%Vpu rise above rated voltage V load = Vrated + %Ipu * %Zpu 

This is because %Zpu and %load regulation error are equivalent.

https://en.wikipedia.org/wiki/Per-unit_system

1 p.u. 100% per unit of rating for V,I,Z,P