It sounds like this data is from two datasources is that correct? In the top left you have two database cylinders one of which has account, country, and sales fields and the other which has country and headcount as fields? I do not think this has anything to do with distinct values.
I think all you need is WINDOW_SUM(SUM(Sales)) / WINDOW_SUM(SUM([DataSource2.Headcount]))
You will also need to use blending at the country level and on the primary datasource define your groups and then set this table calculation to compute using country.
In Tableau 10 you can define custom regions geographically and remove the country level from view. This allows the calculation to work and still see both Aus/NZ at once and would remove the need for the table calc
GroupsExample.twbx 40.8 KB