In general 80% of flare comes from the subject which leaves only 20% coming from stray light. In the black box example, flare added 0.38 more log-H exposure with the white card compared to the black.
Originally Posted by markbarendt
Flare works exactly like pre-exposure. That would be a good way to look at it.
If you look at the two quad example, you'll notice the camera image has two curves. One is a non flare curve and the other is a one stop flare curve. Look where the non flare curve intersects the film curve. Without flare, film would be one stop slower. The ISO film standard factors in one stop of flare into the speed calculation. So, you should assume that you will always have around a stop of flare in most average shooting situations. Higher flare will add more exposure to the shadows effectively making your film faster. A good rule of thumb is 1/3 stop more or less flare per stop increase or decrease in subject luminance range.
Down side of flare is the compression of the shadows.