Index weighting calculation

[abu007]

I am trying to perform an index weighting calculation, using the formula (Value in Cell * Grand Total of Grand Totals)/(Row Total * Column Total). This calculation is a custom measure.

The problem is that when applying a filter to a dimension this calculation is run against, the filter no longer has context for the grand total to work correctly.  What I am looking for is the ability for the grand total to ignore the filter context of the dimension.