MicrosoftI had something a bit more targeted in mind. In the case of the tax bands, 0% was intended to be the tax rate returned, not just some generic default to indicate an error has occurred.
In the case of commission rates it would be possible to work with the upper bounds, in which case the commission scale could be extended by the formula
= XLOOKUP( sales, Commission[Upper bound], Commission[Rate], topRate, 1 )
I got a surprise while testing my assertions, though. I added a band with ">200000" as the upper bound, expecting it return an error; instead it returned the top bonus rate from the table perfectly happily. I hadn't really thought of the case of non-numeric lookup values but this behaviour makes it possible to return NA() or text such as "Invalid sales figure" if the amount is entered as text, e.g. "100000USD".
I have no idea what constitutes best practice but plenty of options seem to be available.
