OK, I've probably been staring at this too long and overlooked the obvious. How do I count the number of related records within a found set? I've set up a calc field with the count function but it returns the number of related records for the entire database. Thanks!

You can filter a relationship by additional criteria, then count its related records. (The found set generally has nothing to do with the records that are related.)

Or

you can create a calculation field which results in 1 if it is related else returns 0.

Create a summary field based on the total of the above calculation field. The summary field would result in the total of all the matching results in the foundset.