When I was looking to get a bull cross a couple of years ago, I looked about for 1/4 bull but would have settled for 3/8. It all depends on how they chuck really. Gameness in the bull sense is lost the moment you cross. I don't think you can fault greys and whippets gameness (different sense of the word this time) but I think the bull adds that desire, that inability to feel pain to the point of loving it, the physical strength and structure that enables them to work fox after fox, night after night.
Personally, I'd go for what I've stated above. I've seen racy half crosses, especially som