A 38-year-old man is having root canal treatment of his upper left central incisor for pulp necrosis with asymptomatic apical periodontitis. The apex locator is not working today, so a diagnostic file radiograph is taken; the measurements are shown in the image. The root is straight, there is no resorption, and the practice protocol sets the working length 1 mm short of the radiographic apex. What working length should be recorded?