Store the data invoice total in a number field and use it in your searches. Happens to be faster too. Just use a script to set this number field to store the total whenever the invoice becomes "final".
The need to search values like the Total for an Invoice, is one of the reasons I switched to scripted Set Field for these types of values. Because they are historical data fields that do NOT change once an invoice is complete, I do not like them to be changeable calculations. If an invoice needs to be revised, it is not complete and the total will be reset (by script) after any changes.
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Ok thanks for the quick replies Phil & Beverly, I can see the benefits of using your method and will look into making the changes.