Assuming that there are no related tables holding data to be summarized in this report, define a calculation field with this expression:
Summary field 1 + Summary field 2 + ....
Is each row a record or a sub summary layout part?
If it's a sub summary layout part such that a summary field placed in the layout part shows the sub total, just put that same exact summary field in your grand summary, header or footer to show a grand total over all the records in your found set.
Each row is in the Body part of the layout with a single row portal to the Estimates table showing the Summary fields.
If I add a the portal to the grand summary part it just has the same values as the last record in the report, and not the total of all of them.
If I take the field out of the portal it summarises all of the records, but not just the ones within my specified date range.
The portal is filtered to show only estimates within my specified date range — I suppose it's this that's confusing me.
That's why I asked if each row was a sub summary layout part. What I suggested works in that case not in yours. Please note that from the beginning that I said "Assuming that there are no related tables holding data to be summarized in this report."
To me, it looks like basing a summary report on the Estimates table and using sub summary layout parts would make for a simpler layout design.
Getting a grand total from the context of a related table will be complicated as your total will be controlled by a relationship (and possibly a portal filter) to match to the correct records in your table. If you need a grand total based on every record in Estimates, this is simple. If you need to match to only a sub set of those records--such as all records dated in a particular range of dates, it's more complicated.
But these are complications that you can avoid if you base your layout on estimates instead of using one row portals to get your subtotals. (and yet, there may be very good reasons for using one row portals--you'll need to let me know if that's the case here.)