Michael R 1974,
Originally Posted by Stephen Benskin
Yes Stephen made a tiny typo, he meant Flare factor.
But look at this chart again. Now imagine doing a single-tone film speed test where you meter Zone V and stop down 4 stops (or 4 1/3 stop if you prefer).
When you stop down, you move everything down.
In the middle of the graph, Zone V, the meter point... you see the exposure+flare (realistic exposure) curve practically rejoins the theoretical exposure (45-degree theoretically perfect) curve. So a test exposure is not influenced much by flare.