Just tried this in Tableau Public and (while it can't handle the full 9-million-row data set), it looks like it runs the measure calculation at the lowest level of detail before aggregating. That seems like the safe, if not the efficient choice, and results in the correct totals.
I'm assuming at the moment that there is no control over this in Lumira, so I guess the solution would be to update my data set with the actual measure value in a new column. Or am I missing something?