Add a “totals” field on header that is sum of lines?

When adding a new “feature”, can you add the ability to create a field that is the calculated sum of a given field on the Lines table for that feature?

I have tried to do this using flow-fields, but 1) you have cannot pick the lines table for the new feature when setting up the flowfield, and 2) even if you could, this would break when displaying the “posted variant” of the new feature.