UV-C devices, such as the Arc, are optimized for smaller, enclosed spaces; those are the environments with the best efficacy. That is why for larger areas, we recommend running multiple shorter cycles in different parts of the room versus one longer cycle in the center to achieve the same kill rate.
The distance to the farthest wall is the most accurate way to determine optimal run times. The table is below for reference. A laser that measures the distance from the wall to Arc comes with the Arc welcome kit. A tape measure works, too, depending on its length.
The room's square footage is a reasonable estimate, but this measure can be more challenging in a room with an irregular shape (L-shaped, for example).