Tysen - Great Question and I will also add this to the Advanced Corridor Modeling Blog as a follow up here

What I would do is use the "Test Shot Approach" (similar to the strap length for MSE Walls concept) which involves taking the Top of Wall and Base of wall points and doing a Slope Slope intercept where the Slope of the line from Wall Top is at 0.8:1 (assumes Strap Length is 0.8x Wall height) and the slope from the base of wall is 0%. This computes you a node X that you dont include in a material layer. Now when you want to create an Offset /Elevation from the Inside Base of Wall which is that 0.8x the height of wall you can use Node to Node for the offset and measure the first node as X and the second Node as the base of wall (outside) point - this gives you the distance - either a positive or negative value - reverse the nodes if you need the opposite of what you got. then for Delta Elevation you enter 0 to maintain the base of wall elevation.

Now in your case you want the following (I think I read this right ...)

So what I would do is

1) create the Finished Grade "Design" in this case I made it Horizontal (red Line)

2) Create a Test Shot at 0.45:1 to hit the Material Layer Finish (you can do this in v5.1) or you can do a Slope Slope from the OG CL at 0.45:1 and from HAL at 0% to compute the node onthe Design to the right of CL. The distance from this node to the HAL is the depth of surcharge

3) Create a node using Slope Slope using Slope= "Up" (yes enter the word up which means vertical) from the HAL and at -1:1 from the first Test Shot you created - this is the Surcharge CL. Now create your Surcharge Material Layer Elements. Note if you want to do a Takeoff of the Additional Surcharge Material you will need to create a third material layer called "Top Surface" that follows finish grade to the tie points Left and Right and that connects from the Tie OG to Surcharge Tie OG to create the closed shape of the Surcharge - because Surcharge is a Material Layer that sits above Finish (Finish doesnt allow you to enter a Material above it, hence create a Top Surface using all the Finish Grade Nodes (Double Tag them as FINISH and Top Surface) and then you can say above TopSurface is Material Surcharge - the enclosed project shows you how.

Hope that this helps - "Thinking Outside Of The Box"!

Alan