Most cats have their rudders and dagger-boards located in the hulls. When turning the cat, its hulls will follow arcs with different radii. Hence, one must consider the fact that the rudders might have to be deflected to different angles to achieve the best turning performance. This is not unlike the situation for cars. The resulting geometry of the tillers and the connection bar is called Ackermann geometry.
Even a purely geometrical analysis results in a situation in which one must make a compromise between the optimal geometries for different turning radii. Tom Speer has investigated what happens when angles of attack of dagger-boards and rudders enter the equations. Unfortunately, Tom does not provide us with a way to calculate these angles.
I have based the tiller geometry of Scarlattikvarten on the results I got from the geometrical analysis. Experiments while motoring indicate that this geometry (tiller toe in circa 30 degrees) is quite correct in this case.