What I would like to do is have a field [Balance] in the [Main] table which calculates all the Gross_Totals for a specific client's jobs where the field [Date_Paid] is empty
There are multiple ways to get that unpaid total. Here are two:
Define an cUnpaid_Total field in jobs as:
If ( Not Date_Paid ; Gross_Total )
date_Paid must be a field of type date. If Date_Paid is empty, this field returns a value, if it has a date, this field will be empty.
Then a summary field defined in Jobs can compute the total of cUnpaid_Total or a calculation field in Main can use Sum ( Jobs::cUnpaid_Total ) to compute the same total. Either field, the calculation from Main or the Summary from Jobs can be placed on your layout for Main and they will show the correct total
Once again I am amazed by the speed and accuracy of Phil's answer.