Not exactly true. The ISO standard has a "triangle" of conditions/parameters to be satisfied in finding the speed point. It specifies a growth in density to 0.8, 4 1/3 stops up from the speed point of 0.1 above B+F. So there is a required gradient up into the midtones. Granted depending on the film and processing the curve over that range may exhibit varying degrees concavity, but by the time you're into the midtones that is pretty much a non-issue.
Originally Posted by markbarendt
In a broader sense, the delta-X method relates the speed point (essentially the basis for shadow point methods) to the fractional gradient method - which was based on print studies, not arbitrary shadow placements.
My two cents.