The scene chosen was an average one giving an image illuminance range on the negative material of 32, which is very nearly the statistical average of a large number of scenes when photographed with a camera system having average flare characteristics.

Note the 32. Log-base-2 of 32 is 5, so this is a 5-stop range on the negative. 5*.3 = 1.5, so this is the source of the log-E range of 1.5.
Note the "average flare". That was for cameras made in 1935-1945, which had uncoated lenses. It seems to me that modern multicoated lenses have less flare, and thus the log-E range of 1.5 should be higher nowadays.

Stephen has said that after accounting for flare, the average range is about 1.8. So if we were to use the fractional gradient method with modern cameras, should we change the formula to this?: Gmin = 0.30*Gbar(1.8)

If so, do you think the 0.30 would also change?
Nelson touched on this in Safety Factors in Camera Exposure. Lens coating shifts it back 0.05 log-H units. This simply introduces a small safety factor. In fact, I believe this might be what is at least partially responsible for the ASA / ANSI standard's note in the forward about a built in 1/3 stop safety factor.

From Safety Factors, "As point out by Jones and Condit, however, the shadow point should coincide with the speed point only when the flare factor is 4. When the flare factor is 2.5. the deepest shadow can be placed about 0.05 to the left of the speed point because a lower slope on the toe of the curve becomes usable when the shadow contrast in the camera image is increased by the reduction in camera flare. Consequently, the "first-excellent" point in Fig 3 is considered to lie 0.05 to the left of the speed point. The "first excellent" point, therefore, lies 0.37 in log-H units to the left of the shadow point, c, representing the exposure obtained from the use of the exposure meter and the ASA exposure index. The interval of 0.37 is the logarithm of the safety factor. These calculations, therefore, lead to the conclusion that the safety factor is 2.35" (for the fractional gradient method)

So basically, yes I believe the factional gradient value could be lower with coated lenses. You can also see that 1.80 was being used when considering the change from fractional gradient to the fixed density / Delta-X method.

Safety factors.jpg