How is the user's selection being selected?
In a dashboard a selection can drive an action. Actions can be either filter, highlight, or URL. In this case, I think you will be able to accomplish what you want with a filter action. Based on the user selection, the target view would be filtered and the calculation will be based on the result of the filter.
There may be additional complexities depending on your situation. I'd be happy to take a look at a sample packaged workbook (with mock-up data) if you get a chance to post it.