If I decoded that written description, you have or need these relationships:
(It is possible for one invoice to include items sold by different salespeople.)
If you set up the match fields between Employee_location_Days and Invoice Line Items like this:
Employee_Location_Days::_fkEmployeeID = Invoice_Line_Items::_fkEmployeeID AND
Employee_Location_Days::Date = Invoice_Line_Items::SaleDate
An aggregate function in Employee_Locatoin_Days or a summary field in Invoice Line_Items can show your sub totals.
SaleDate can either auto-enter a date from Invoices or the creation date to autopopulate this field.
An alternative method that can be used with versions 12 or newer is to use ExecuteSQL to compute such aggregate values.
Phil, thanks. That was very helpful. I set up the relations as you suggested, and used the Sum function in a calculated field in Employee_Location_Days. Works great.
In my 4D days, it was preferable to use automatic relations only in a very limited fashion, and to accomplish the more complex things programatically. In Filemaker, the relations are much more central to the operation of the database, so it's taking me a while to understand and embrace the utility of compound keys, table occurrences, etc.