When development according to ISO conditions, then you measure the Average Gradient.

0.3G is 0.29 left of the .1 density because it is expected to be there... do you start there then look 1.50 to the right for the other end of Average Gradient.

Or when you do average gradient, do you pick two points 1.50 apart according to other instructions (again this will happen to be on a curve that meets ISO conditions). Then the 0.3 Gradient is not required to be one of the endpoints.