Your iif statement looks good to me; I wouldn't worry about changing anything. Just as an FYI tho, there are other ways of doing it, for instance:
You can create a calculated field in your query using the info in the link below
Then put a parameter in your Criteria line for that new calculated field. You can also have the query prompt the user for criteria by using brackets  in the criteria field. for Instance: [Please enter Value] will prompt the user for a value and use it as part of the query before it runs.
I hope this information helps!
Law of Logical Argument: Anything is possible if you don't know what you're talking about.