I found a 'your gifted child' book lying on the table at borders today so had a good rifle through it and lo and behold I found some definitions
So...
Gifted the 'top' 200 out of a thousand
Very gifted the 'top' 100 our of a thousand
Genius the 'top' one in one thousand.
There was a very interesting discussion of 'top' but the consensus seemed to be that it was potential intelligence that they tried to measure and there was a few chapters discussing how this might be measured.
Interesting to note that the book was from the clearance pile