QuickBase formula are quite robust and are capable of taking quite an abuse :D
You can use an excel to list down all the conditions and build 1 single giant formula. Your current idea of using dropdown in 1 single field should work fine.
It will goes like
If (
[Location] = "X" and [Supervisor] = "Mr. A" and ["Manager"] = "Uno" and [Product] = "12", 1.24%*[Sales Commission],
[Location] = "X" and [Supervisor] = "Mr. B" and ["Manager"] = "Uno" and [Product] = "12", 1.22%*[Sales Commission],
[Location] = "X" and [Supervisor] = "Mr. C" and ["Manager"] = "Uno" and [Product] = "12", 1.04%*[Sales Commission])
You can continue to build like above in 1 formula with 50+ combos easily , it's easy to manage and you can make changes .
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Another way would be list down all the combinations in a child table and then doing a parent child relationship to manually select the right combination. If there is a business case to have history of previous commissions or changes , this would be a right approach .