Damn math! Sorry, I got a little carried away trying to make the connection. How about almost identical to the fractional gradient method.
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.
With fractional gradient you need to guess a point in the toe, then calculate the average gradient, then check the toe gradient. Repeat as needed.
Under the ISO contrast parameters, the fractional gradient point is there 0.29 log-H to the left of 0.10 density. You only need to do the 1.30 / 0.80 measurement from the 0.10 fixed density point.
Working backwards then, find the point 0.29 log-H left of 0.1 of the film-developed-to-ISO-conditions curve. Find slope of the tangent to that point and multiply by 3, do 1.50 times that, to find the density delta. Make a triangle with that density and 1.50 log-H and find where that triangle meets the curve.
What will I find? Will some of the measurement points coincide or be in a predictable place?
Or is it "Not locked down" and the Average Gradient triangle could wiggle anywhere depending on the shape of the toe?
I've never tried it. I have found that long toed cuves have a slightly higher CI index when matching the ISO parameters than straighter curves. This make me assume there won't be a perfect match with the 1.50 fractional gradient approach all case. Remember the graph of the spread function? But neither does the fractional gradient method always perfectly conform to the print judgment speeds.
I'm a little reticent to post since I've muddled my words pretty bad recently, but I don't understand Bill's question. Are you essentially trying to figure out if Delta-X reconciles the triangles on the speed point side such that certain characteristics to the right "line up", when the ISO criteria are met?