They are treated no different. Horns are just waveguides (and yours are more like traditional waveguides on say a studio monitor rather than a true horn on like Advantgarde), and all they do is focus the far off-axis sound emission forward, giving you a wider soundstage and some added output.
Since Cardas isn’t asking your listening distance, it isn’t calculating for first reflections, so I would use the distance directly perpendicular to the speakers.
Since Cardas isn’t asking your listening distance, it isn’t calculating for first reflections, so I would use the distance directly perpendicular to the speakers.