To get a summary value from a filtered portal, do this:
Define a field of type summary in the portal's table. Specify the options you need for your aggregate value (total, average, etc.)
Make a copy of your filtered portal but change the specs so that it displays only one row of data. Put the summary field from your portal table inside this portal row and place portal and field where you want it on your portal. The key is that both your original portal and this one row portal must use the same portal filter expression.
The other option for getting subtotals for different groups of related records is to set up a summary report based on the portal's table with fields from the parent table added as needed to this same layout. Sub summary parts and summary fields--with the correct finds and sorts can then group and sub total your data as needed.
Here's a tutorial on summary reports if you are interested: Creating Filemaker Pro summary reports--Tutorial
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