How's it different than the resulting densities from a step tablet exposure (other than way less accurate in the exposure increments)? I still think you're over thinking this. Maybe if you walk through the details of Henry's procedure and point out the part you are having a problem with. And BTW, Henry's concept of speed and metered exposure isn't exactly on solid ground, but that isn't related to the problem you're having.You stopped down 6 stops below metered. It may be that 5 2/3 stops below metered also yields zero net D. Unless you maintain the metered exposure as a reference point, I don't see how you can plot your first non-zero net D exposure at 0.3 relative to a zero net D exposure.
Film Curve example 2.jpg