Add a Sub Summary layout part to your layout "when sorted by Category".
Put a summary field in this sub summary layout part defined to compute the "total of" your Area_Cal field.
Be sure to sort your records by the Category field or the sub summary part will not be visible.
Here's a tutorial on summary reports that you may find useful: Creating Filemaker Pro summary reports--TutorialCaulkins Consulting, Home of Adventures In FileMaking
Do I still need to create a field to just do a total of the departments I want? When I followed your instructions, it looks to total by the category of all depts/areas associated with that category. My total that I was looking to get (from the image provided) would be 2,918 for this one store, but I would want this similar total to show for each store, which most would have a different total.
If I'm missing a step, please let me know.
You haven't yet placed it inside a sub summary layout part.
The same summary field, if placed inside a sub summary layout part, will display the sub total over all records of that category. Put this summary field in a header, footer or grand summary part and the very same field computes the grand total shown.