You could use Dimensionality() / SecondaryDimensionality() to check in your expression if the evaluation takes place in your partial sum.
So you could branch your expression into an expression that gets calculated on detail and one that gets calculated on total cells.
And if you need to ignore some selections, you could use set analysis.
This may sound not really helpful, but with the information provided, I can't offer more specific help.
If possible, post a small sample QVW and describe what your expected result is.
