Hello
I'm not sure but, for what I know, you just need to sum 3 stars in each position with all your actresses. Obviously, if you have an actress with 3 stars in all position, it's better but not necessary. The number of stars you put in each position can be seen left in the windows of selection.
Once you complete some positions, the atmosphere, the sex appeal and the ethnie became more important. It's even possible that, after completing the casting, if you retire the first actress selected (that gave a lot of % because of her positions), the game propose you another girl as "better" to fill the empty spot. To optimize your score, it can be usefull to retire each girl of the casting individually, just to see if the game propose her again with the rest of the girl
Hope it can help you^^