AutoCAD Civil 3D :: Stage Storage With Depression In Bottom

What would be the best way to create a stage storage report with a basin that has multiple depressions and high points in the bottom of the basin?

The stage storage report seems to calculate these features as an addition when it should be subtracting for high points.

Could I do a bounded volume and set the datum to each increment of storage? How accurate is that? If it's accurate, is there a way do generate multiple at a time instead of typing D then setting the datum, then repeating that process over and over?

The only way that seems the most accurate is to create multiple planes at different increments, add those to seperate surfaces, then a volume surface for each one comparing it to the basin. This method takes up a ton of time, especially when dealing with 50+ increments.