The term Lighting Ratio describes the difference between a main light and any fill light and this relative strength can be easily measured with a light or flash meter. In camera meters measure the light reflected by the scene being photographed. Hand-held incident light meters measure the amount of light falling on the subject. Spot metering measures a narrowly focused area in a scene but still measures reflected light.
If you have a meter reading for a subject of 1/500 sec at f/11 and want to use a slower shutter speed to allow for normal subject motion and choose 1/125 sec you will have to adjust the aperture (make it smaller) so the same (equivalent) amount of light falls on the sensor. By selecting either Aperture or Shutter priority exposure modes, your camera calculates the equivalent exposure for you, eliminating guesswork.
The Brightness Range method suggests taking two readings different from the scene you’re about to photograph. The first one (or second if you prefer) is a meter reading of the highlight area where detail is desired. A second reading it taken from the shadow area of the scene, again where you want the detail held. Your camera settings will be based on an average of the two readings and may be close enough, although you might try to bracket your exposure. For information on bracketing, see “If all fails, bracket.”
Exposure with Canon EOS 60D with EF 28-105mm lens was 1/125 sec at f/7.1 and ISO 200. Image converted to monochrome with Silver Efex Pro.