But how can I get the number of shops who failed to meet at least 80% of planned sales? I tried some stuff with Set Analysis, but as I'm new with this, I don't know if this is right, but I do know it doesn't work:
You can see this if you create a chart with Shop as dimension and the above expression. Make sure to uncheck 'Supress Zero Value' on the presentation tab. Now you will see a bunch of 0s and -1s. All those shops which have -1 will be shown by your main expression and the shops with 0 won't.