If I have the MRF-200, do you have a value in mind for the resistance?

Sometimes LEDs are driven by a constant current driver and adding a series resistance would only make the driver put out more voltage to compensate for the resistance. Do you have any idea if this is the case? (Now would be just the time for a very nice HTM guru to pop in). If that is the case, it might make sense to have a parallel resistance or series / parallel.

I have seen the resistance idea tossed around, but never heard what a good value would be.