I suppose if I worked at it, I might get this to work, but i strongly recommend that you not use repeating fields to list individual line items.
You'll find it much simpler to use a related table of line items in a portal. And then a calculation field can return the line amount for each item that is taxable so that this calculation field can then be summed to correctly compute the tax.
Agreed. Life would be a ton easier I didn't have to fix legacy systems and worry about backwards compatibility.
I've read that repetitions can be split into individual fields as an option during export. Haven't tried it yet. Not sure I want to go that far to fix a little problem that everyone has just worked around for all these years.
When you IMPORT data from one filemaker table into another, you get a check box you can select that will spearate the contents of a repeating field into separate records. This can usually make dividing your data during import into a table you now set up as a related table of line items fairly easy to do.