Hi Alix,
I believe you’re almost there with that formula, but rather than using a SELECT, try using a BY CONSTANT. You would never use the SELECT modifier followed by a dimension-type metric. If in doubt, use BY!
(Take a look at the SELECT and BY modifier community articles if you would like more information on them.)
If you’re not using it already, Formula Playground is a great place to develop your formulas when you’re not sure exactly how they need to be written. You can write a formula and, if you are in ‘Automatic structure’ mode, Formula Playground will give you the inferred dimensionality of the output metric. This can be useful when you’re not sure which modifier to use.
Hope this helps. Please let me know if you’re still having trouble.
Cheers,
Matt