Consider a second field (column) showing the taxable amount (max allowed or the actual amount if less using query calculated field and IIF expression) then a 3rd that calculates the tax based on column 2, then sum only the tax values. Wouldn't that make it clearer to everyone what the tax sum is based on? Otherwise, someone is applying 6% to the full amount and wondering why it doesn't jive?
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.