You can place more than one field on your label mark so that you can have the metrics on top and the 30 days variation underneath. For the Last 30 Days variation, how are you calculating that? Is that the 30 day period prior to the filter? Or the last 30 days in the filter? Or something else?
If You are looking to fix a date and want to go 30 days back using that, you need to create a parameter and your parameter value and your selected order date values when match, you may create a Calc to get 30 days back data and show.