Sum A Formula

[wiinc2008]

Hi -
I am trying to create a total (summary) field for a report that I am creating in Crystal Reports XI that is based on a formula field. After pulling the formula field into the report, I clicked on the sigma symbol (circled in red). After doing that a box comes up (in yellow) that asks which field I want to summarize, but does not provide my formula field as an option, just the fields in the different tables.

Any advice on what I am forgetting to do in order run a total or average of the formula field? It has been about 3 years since I used Crystal Reports and I have forgotten a ton about the app.

Thanks in advance for your help.
Brian

See attached for the screen shot of the application and the fields referenced above.