I'm not familiar with a Butterfly, but as long as you increase the dihedral you won't need to bend the outer sections up. I've never flown a rudder-elevator that only had dihedral, but as a rough guess above 10 degrees should do.

Of course, you could always break out the ol' trigonometry and calculate how much dihedral you'd need so that the tips are in the same location as in the plans.

Dan Nelson