I don't have the correct version of tableau to open your dashboard. I tried this using sample store data.
I created 2 sheets
Source sheet is a map view with all states(49 unique states in the dataset)
Target Sheet - Where we show the %Highlighted.
Created a calculated field
and used it in sheet like this
and created a dashboard with the 2 sheets. Added an action with Map as source and the other as target sheet.
Now based on the no.of states selected, the %selected sheet displays the required value.
Hope this helps.