WIth rudder application the geometric up that is mixed in is cancelled by a similar down mix on the other side. V tails work this way and your canted fins would do the same thing. So set them to any angle you like that goes with the looks and you won't have any problem or need any mixing at all as long as both sides move the same.

Just be sure that you have enough effective vertical area to provide the stability you need. Perhaps make up a little flat sectioned profile all sheet test model of around 12 to 16 inch span. You can tell a lot from how such a model flies even if it's unstable from a lack of dihedral. But pitch and yaw stability is easy to see.