Um, according to Un3x these valley reports show a bonus that is NOT inline with the current idea that the lowest star determines the bonus. These tests show a curve in the bonus.
http://www.inetassociation.com/evony/starset-a.jpg
http://www.inetassociation.com/evony/starset-b.jpg
http://www.inetassociation.com/evony/starset-c.jpg

Star set A lowest star is 1, I get 0 star set bonus
Star set B lowest star is 8, yet I only get 5 star set bonus
Star set C lowest star is 10 yet I get a 9 star set bonus