According to the manufacturer, "Lightning flashes in the Alarm category are most likely to have occurred within 5 miles of the Sensor. Lightning flashes in the Warning category are most likely to have occurred within 10 miles of the Sensor. Lightning flashes in the Caution category are most likely to have occurred within 20 miles of the Sensor."
The SG000 is not capable of determining the exact distance to a lightning strike. Determining the exact distance would require multiple sensors and a triangulation method using TOA (Time of Arrival).