I got it working. I was having trouble understanding most of the suggestions and plugged away on my own until I came up with this:
Subquery2 has one calculated field with just one result:
Then the main query has a calculated field which pulls out just the value from Subquery2:
Then the dashboard which uses this BAQ filters based on that calculated field. Maybe not the most elegant thing but it is the only thing I could figure out.
My first ever subquery! Thank you everyone for the pointers!