Hi, it would be generally easier to calculate this in the cube after the import. I would try going that way.
Otherwise, you can use a business rule or complex expression in your derivative rule to pull the data you need from the corresponding table. If you go down that path, using Globals is highly recommended for performance. Otherwise, pulling the source data from other tables will (likely) degrade transformation performance notably. This can be done (without knowing the exact details of your requirements), but if you can calculate this in the cube as initially stated, that would be easier in my view.
