Consider if the subject in the foreground was holding a piece of the white background seamless paper next to their face with only the key and fill lights on. When the exposure for the face was correct the paper would be reproduced as white wouldn’t it? Not 255.255.255 blown out white, but something in the range of 240s which makes it look like what it is, a piece of paper not the blazing sun.
Let’s say you measure the key and fill for exposure by pointing the meter at the camera dome-up and get a reading of f/8. Now leave the key and fill on and put the meter on the background near where the head appears in the viewfinder and take a reading.
