I agree, zero may not relate by 0.3 from lens cap zero to Zone I. Now it would relate if you created a Zone 0 exposure, but often the flare doesn't allow camera tests to go that low.

I'd be more likely to try to estimate the exposure at the x-axis where you find a net D of 0.1, and start my labeling from there. I figure if you hit the ASA parameters with a standard developer this point is likely to correlate reasonably well to the manufacturer's rated speed.