Pass through specific filters via tooltip

I have a report for KPIs with data stored against Areas: National, East, West, North, South
I want and have built the front page to show an Area (defaulted to National, but the user can specify another Area).

I've worked out i could write a DAX code to show the National figure on the front page, but i think this would restrict the user being able to filter to their zone. Currently i've built the page with a slicer on Area set to National as default. (If relevant, there is also historical data controlled by a filter to default show most recent month of data).

I want the tooltip to show that KPI for all Areas to provide context. 

I can either pass through the KPI AND the Area by using 

or, i was hoping i could use "tooltip field" to specifiy ONLY the Key Performance Indicator gets passed through. But this seems to have no impact and shows all KPIs and all Areas.

something like this:

With "All Filter" selected.

With the specific tooltip set to KPI

Hopefully this makes sense, i've tried to anonymise my dataset but keep the principle,

Many Thanks,