This one comes up from time to time  how do I determine where my pond capacity line is for e.g. 20% and 80% full.

The key is to create your Full Capacity line and create a surface from it  call it Pond Top

Then determine your volumes at e.g. 0.05’ intervals using the Earthwork Report

Copy the report values into excel and create a cumulative column and sum up the band volumes to each elevation interval

Now take the cumulative total and determine what 20% of the volume represents

Now take the cumulative volume and determine what 80% of the volume represents.

Now look down the cumulative volume column to find the 20% and 80% volume numbers (or the closest number to those percentage points. These will be your two contours that you need to create

Now use the Create Contour at elevation, and enter the specific values that you determined from the list. Name the first contour 80% and the second one 20%. These contours are your capacity lines.

If you need to check the numbers, explode the contours to make them 3D lines. Once they are 3D lines you can create a surface for each one  call the surfaces 80% and 20%.

Now you can see those surfaces in the Surface Slicer and 3D Views.

Now you can run the Earthwork Report between each of these surfaces and the Finished Grade and the results should match the initial volume report.

If everything jives, these are the lines you need for the staking operation in the field.
Video shows you how