Your Flash Meter is Your Best Friend

Electronic flash is the preferred lighting medium for professional
photographers. They come with strength ratings, color balance figures and watt second figures. But when all is set up and ready, the only number that will lead to a perfect exposure is the one on the flash meter.
Every flash tube has a signature and the output varies with time and the number of flashes that it has produced. The older it is, the weaker the flash and the more yellow in the light. Reflectors also change characteristics by becoming dirty or by varying the distance from the tube.
Cameras and lenses also affect the final effective exposure in the image file. The fewest artifacts are visible when the lowest ISO rating is used in the camera. Use the flash meter to set the power level on the main light to the F stop you are using, i.e.: F11. Then meter the hair and back light to the same F stop. The fill should read two stops weaker than the main. Using an 18% gray card, make a test exposure and read the histogram. The gray card should spike about in the middle of the graph. If it is to the left or right of center, adjust the ISO setting on the meter to correspond to the center of the histogram.
It is best to check the average flash intensity at the subject location before starting the first session of the day. An occasional check during a busy day will ensure that the exposures are dead on for every session. Perfect exposures are necessary to produce good skin tones and natural contrast in your portraits. There are enough challenges going on in a portrait shoot without worrying about exposures in your digital images.