I've added a few things to one of the two quad examples from post #201. This example has it's exposure based on the ISO. Zone V represents the metered exposure. As the example shows, the equation of metered exposure is 8 / ISO and the example uses a 125 speed film so the metered exposure is 0.064 lxs. Speed point falls Δ 1.0 log-H units below at 0.8/ISO or 0.8 / 125 = 0.0064 lxs. So in this example, the exposure was perfect for a film that has an ISO of 125. Zone I falls below the speed point at 0.0041 lxs.