When the manufactuer determines the transformers impedence, the transformer KVA rating is used in the calculation.
So if you are only given one transformer impedence, you should use the transformer KVA rating that was used to determine that impedence. I don't know what the standard is, but if I had to guess, I probably guess that the standard OA KVA rating is used to determine the %Z.
If that's true, there wouldn't be any need to used the forced air transformer KVA rating.
But are the AIC ratings of your OCP devices really so close to the available fault current that it would make a difference which KVA rating you used??
Steve