This paper describes a practical courtroom tool that provides insight into familiar face recognition accuracy as a function of distance and illumination. Subjects were shown pictures of faces of famous persons or lookalikes, with an exposure time of 12 seconds. Subjects were asked whether the person shown was familiar or unfamiliar to them. If familiarity was indicated, subjects had to identify the person. Seven distances (3-40 meters, i.e. 10-131 feet) and nine illumination levels (0.3-3000 lux) were combined, which resulted in a large matrix, each cell containing a hit score and a false alarm score. From these raw data diagnostic values were derived. The results show a systematic increase of recognition performance with decreasing distance and increasing illumination. The diagnostic values suggest a practical rule of thumb, the Rule of Fifteen : even in ideal conditions the desired diagnostic value of 15 is reached at no more than 15 m (49 ft), and no less than 15 lux.