There is no such thing. You can either shoot a bow well or you can't. You might struggle with a particular bow because of its design and, yes, brace height may have something to do with it. But I've shot plenty of bows with very short brace heights and shot them well. Conversely, I've shot bows with longer brace heights and couldn't get them to group for anything. It's more about the bow's overall design than it is about one singular measurement.