Can you attach a packaged workbook?
I would do this in the data source but if you don't have that option I can try to figure something out.
I think you can use "Bins" mechanism to get your required result.
Assuming I am understanding your goal correctly, it sounds like a good example of a Slicing by Aggregate problem -- see this link below for several approaches that may help:
Note that this should be easier to accomplish with Version 9 LOD calcs, as demonstrated by Joshua in his blog.