Since the purpose of the grey is to prevent burn in from creating vertical stripes, the 'grey' should ideally be the time-averaged value of all the image pixels when showing a selection of typical programmes. I have no idea what that value is - and it may not in fact be grey. Not also that 'ideal' may not be good as aesthetics are important to. The value used may be dimmer than ideal.

I assume that you want this information for an ATI software product. You could possibly add some code to the card driver to collect that statistic.