I think there is some misunderstanding here on what the MeasureName field is doing. All it contains is a list of the Measures that you see in the data source part of the design window (excluding MeasureValues and generated Long/Lat fields). This means that if you only have one Measure, your calculated field, that is all you will see.
The MeasureNames does not have "Only Relevant Values" because it is looking at columns, not rows, in your data. The columns are always present, just not always used, so there is no such concept as "relevant" columns.
What are you wanting to show in the Measure Names?
When you choose a Region, what are you wanting to see?
If you could post a mock-up, or even better, a packaged workbook, that would allow us to guide you further.