The vestibule is actually rectangular, but the exterior door is 90 degrees off of the interior door, so the walking path is not straight. The entire vestibule has walkoff carpet tiles and is roughly 10'-3" x 6'-6".
In measuring the walking distance between the two doors (and therefore checking for 10' compliance), I have three options: (1) measure straight diagonal line from (center of) exterior door to (center of) interior door (walking distance = 9'-5"); (2) measure "L-shaped" walking distance from exterior door to interior door, as if only robots use the entrance (walking distance = 12'-9"); or (3) measure the arc length of a curved walking path from exterior door to interior door (walking distance = 10'-1").
Which measuring method is most appropriate?