Currently, there is no way to determine how a subtotal or grand total is computed. It will use whatever aggregation is on the measure, as you have found. The best way around this is to find the desired values and placed them on a dashboard with the detailed sheet. This is a good one for the ideas section, if it's not already there!
There are methods to achieve what you are looking for. Although, it does require an understanding of how Tableau computes with table calculations.
See the attached for an example.