Flare makes it impossible to determine where the shadow exposure will fall. If you look at the very bottom of film curve in the two quad example, you'll see a Delta X notation. That point indicates the fractional gradient speed point (found using the Delta-X Criterion). The fractional gradient point is the minimum point of exposure that will produce an excellent print (determined from the Loyd Jones First Excellent Print testing).
Originally Posted by markbarendt
Even if it was possible to shoot a scene and experience zero flare, the exposure will still fall at a point where an excellent print is still possible. This also means that there is about a stop safety factor built into the film speed standard.
As flare varies considerably depending on the tonal distribution of the scene, the idea of film speed and proper exposure is about finding a placement that allows for the variance without sacrificing quality. There's a misconception with some that there are specific densities for specific "Zones." According to Jones, negative density isn't an important factor in the determination of print quality. The critical factor is gradient.
You've probably seen this graph before, but this is an good time to review it. It shows the relationship between the negative exposure and the perception of quality in the finished print. Point A is the First Excellent Print which has it's exposure at the fractional gradient point.