NGR to be Put on Transformer

Well this first begs the question, what role do you play in this installation? I hope that you're not the authorized Electrical Engineer or installation firm responsible for the Engineering of this installation. It seems to me that your customer is trying to have this installation done without paying for a licensed Engineer or Electrician approving the design. I believe that the Neutral Ground Resistor value comes out of the electrical code standards for the installation site. (I was tempted to ask how a National Guard Regulation mattered here.) I presently do not have access to my NEC code book to verify this but it might not apply to your location anyway.

The NGR rating is arrived at by dividing the phase to neutral voltage by the desired earth fault current. It is geneal practice in transformers to limit the ground fault current to the rated full load current of the trnsformer (though this is not mandatory).

First of all, you should calculate fault current for the said transformer, depending upon the % impedance of transformer. Now, value of NGR should be such that it limits the fault current to full load current of that transformer.