Based on what you wrote in #1:

The problem is detail row lines may be
deleted and/or inserted.

and

is there a way to allow the summary line to:
a. always display the correct values, and

I thought that you could use one of the examples in the uploaded file. The three examples correspond to (1), (2) and (3) mentioned in #2.

If none of those proposals are of any help to you, please explain better.