Summary: While in your project's sketch scene, click "Facets" in the upper toolbar, apply the correct pitch label to the low-slope section, then apply the "Low Slope" label (located in the Facet Labels drawer.)
Watch the video below to see the process:
Note: Applying the Low Slope label to a facet subtracts that facet's area measurement from the Total and Actual Squares in your project's measurements. The low-slope area is accounted for separately in the Measurements tab of the project, and on the project's Sketch Report.