calculate measure on specific dimension using calculation contexts

Hi,

In my report I have the dimensions "customer" and "country" and the measure "amount".

I want to show in one block the country with the count of the customers.

Additionally I need a report filter to filter only the customers with a amount>0.

How can I implement the filter, so that it works in the block with the country-dimension? Somehow I have to tell the filter that it should be calculated on the customer-level. I guess it's possible with calculation contexts but I don't know how.

Any ideas?

thanks!