How do you convert percent impedance of transformer to impedance value?

The apparent impedance looking into a transformer from one side will not be the same as looking in it from the other, which is why percent impedance is used.

If you are looking from the high voltage winding (I'm labeling #1) to the low voltage winding (#2), you must scale the percent impedance as follows:

(% impedance) x (Winding #1 nominal voltage)^2 / (transformer base VA)