One way to do it is to create a separate calc that returns 1 or 0 according to your criteria. In your example, you'd change you calc to:
IF [Percentage of Coverage]>= 5.0 AND SUM([VehicleCount] ) >= 8 then 1 else 0 end
Then you can create a WINDOW_SUM function to operate on that numeric measure.
Take a look at the attached for an example using Coffee Chain data
eliminate duplicates.twbx.zip 296.0 KB
Solved the problem nicely, now wrestling with data blending...