I know that the desired shape here is quite specific: a more rounded domed center that morphs to more pinched corners.
With NURBS construction there are so many ways to achieve surface configurations, each having their own character.
Though, the examples I give are more rounded on the corners, some of the possible construction methods may hold just as viable:
http://moi3d.com/forum/index.php?webtag=MOI&msg=4388.107
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