Guide number
The guide number is a value indicating the strength of a flashlight. According to Andreas Feininger the guide number helps finding the right aperture for flashlight exposures with the formula
aperture= guide number
distance between flash and image subject (in meters)
Since different manufacturers gave guide numbers for different basic ISO filmspeeds Feininger suggested the user to find out the guide number himself by making 3 test exposures. The image subject must have the same distance for each test exposure, distance and aperture must be notified. The aperture for the first test photo must be selected according to the formula. The further exposures shall be made with aperture below or above the calculated value.
The best of the three pictures will give the guide number
guide number = aperture * distance between flash and image subject