Using ouput data variables in subsequent calculation within ArcGIS ModelBuilder?

Using ArcGIS Desktop 10.3.1, I have two simple polygon shapefiles, each with just a single feature.

I'm new at ModelBuilder, and have 2 tasks:

1. calculate the acreage within each of the two shapefiles, and then
2. calculate their relative size; that is, simply divide one shapefile acreage by the other.

Here's a screenshot of my model:

I successfully completed step 1 by using the Python expression !shape.area@acres! within each Calculate Field tool. The correct acreage is added to the just-added field in each shapefile.

But the model returns the syntax error 000539 at step 2, which confuses me. The ESRI documentation says that the output from each of my two Calculate Field tools in Step 1 is a variable, represented by the name of the green output oval. So I would expect that using this expression in the Calculate Value tool would correctly give me the relative acreage: %pss_out% / %wetlandA_out%

Here's a screenshot of my Calculate Value tool:

Once I get this problem solved, I plan to populate a separate table field with the output from the Calculate Value tool.

In summary, how do I divide the output from one of the two Calculate Field tools in Step 1 by the other?