The luminance range of the standard model is very similar to the Zone System's. In fact, it has 1/3 stop deeper shadows. So instead of the ZS 4 stops, the statistically average luminance range as 4 1/3. According to the Delta-X Criterion, the fractional gradient speed point falls Δ0.29 log-H below the 0.10 density point. This is where the shadow exposure for the standard model would fall in a no flare situation. With flare, the shadows are pushed up to around the 0.10 density point. The ISO standard and Delta-X Criterion use 0.10 density as the speed point. Mostly because it is an easy to find point of measurement, but partly because this is where the shadow exposure will fall. As the ZS has practically the identical luminance range below the metered exposure point, it would go to reason that the shadow exposure will also be brought up to ~0.10 under normal shooting conditions. The ZS test is practically flare free, so the exposure four stops down from the metered exposure point will fall 2/3rd stop below 0.10 density. By increasing the EI 2/3 of a stop, in the no flare testing, exposure then falls at 0.10 density, but with most general purpose developers, if the EI was left at the ISO and an average scene was shot without exposure compensation, flare would bring the shadow exposure up to around 0.10 density.
The example shows just that. The no flare curve's shadow exposure (Zone I) falls at a density of 0.04. With a one stop flare factor, The Zone I exposure falls at a density of 0.12.
Zone System vs ISO 2 Quad copy.jpg