No, you are not missing anything--creative solution you came up with but no, calculated variables from a responses.xls file can not yet be embedded within the parameter values field. It's a great idea though and I will add it to our to-do list. The only work arounds i can think of would be:
  1. a custom item (let me know if you'd like elaboration there or see the most recent user's guide).
  2. (and I like this better)--multiple versions of the same ranged question--back to back within the same que file--with the different range possibilities, e.g., the first has (r1-1), the second has (r1-2), and so on). You would use your responses.xls variable to *skip* to the appropriate one. Then no matter which range version of the question they get, you have them *skip* past the others to the next question. Voilà! Is that a possibility? Admittedly, a pain (esp if you have many instances of this type of question) but probably much easier than a custom item in this case.
For more info on the latter work around you might search the users guide for unconditional skipping (as a way to get in and out of a block of questions where only one is administered).