summing a related field
So far I used count of in found sets summary fields with no issues, these are doing simple counts. Now I need to sum the miles. However if I put a summary field in the Activities tbl, it only counts it once, this makes sense as it's only entered once. If I place it in the VolunteerActivities tbl, there is nothing to sum, no miles in that table. I don't seem to get an option to sum a related field. So it seems summary fields can only sum data in it's own table.
In this very simple relationship, how can I get a sum of the miles a Volunteer has travelled YTD or Total miles for an activity and use it in a sub summary part?