How do I lock a custom metric to a specific hierarchy level?

Hi,
I have three levels of hierarchy: national, regional, and city. I have a metric which is in top box format. I want to show the metric in "regional" and "vs. national" format in the same table. I though I could have a custom metric that is locked to the national level, and a custom metric for each region that captures the difference between the national and regional level. But I cannot find a way to lock custom metrics to a specific level. Any suggestion on how to do this? I'd also appreciate any other ideas to achieve the same goal. Thanks in advance!