Take a meter reading off your main light on your model. An example is F11 (shutter speed remains constant). Once you have the back ground light that you like have the model stand there and take a spot meter reading over one shoulder of the model of the background. Important: Take this reading from the angle you are shooting from! Note value. An example is 4.5. This is a 2.5 stop difference. Now note this difference and which shoulder you took the meter reading off of and you will now be able to recreate this background look.
Tip: Always aim your light meter towards the light source. This will ensure you get proper exposures.
Tip: Keep eyes centered in sockets
Tip: Model head on the viewers right side of frame looking to the left of the frame. Put words (copy) on the left side of the page and the viewer will look at the copy then to models eyes then to the copy. Just as we naturally read and look. The viewer will be forced to look at the image twice.
If the models head is on the left side of the image (as the viewer looks at it) and the models head and eyes are looking to the right the viewers eyes will naturally leave the page and go to the next page.