Two few info to figure out.
Hi Floris. Pretty straightforward - create a Parameter with a list of Measure names (for your different Rating measures).
Then create a calculated field that is based on the current value of the parameter (using a CASE statement). Sample TWBX attached.
Also - here is some more info from the online hep in re working with CASE statements:
Floris.twbx 469.1 KB