What you have now is analgous to telling FileMaker what is the sum of 5? And the sum of 5 is just that: 5.
Define this calculation in the parent table--the table specified in Layout setup | "Show records from" for the layout in your screen shot and it will compute the correct total as then it will refer to the sum of all records in the related table, not just the current record.
Is this what you're referring to? If so, I believe I have it already set up that way.
FCMPCheckbook is the name of a TableOccurrence "box" found in Manage | database | Relationships. It links to another table occurrence that is refered to in "Show related Records From" in the portal set up.
When I look at your first screen shot, it states: From the context of FCMPSSupportingDocuments and your calculation is:
Sum ( FCMPSSupportingDocuments::CostSummary_WithCoop_88KRem )
There's no reference to FCMPCheckbook in that first dialog. If it said "from the context of FCMPCheckbook"--which is only possible if you define the field in the data source table for that table occurrence, then your calculation should work.