Use the GetSummary function, using the summary field you want the total of as both the summary field and the break field.
Thanks for quick response, but I'mnot sure how to use GetSummary. If I explain what I'm doing, I hope you can help me:
I have used the script below to create a report (works fine)
Go to Layout["Float Report" (Table)]
Enter Find Mode
Set Field [Table::Kg;">0"]
Set Field [Table::Date; Table::gStartDate & "..." & Table::gEndDate]
Perform Find 
Sort Records [Restore; No Dialog]
The report has sub-summaries by date ("Total Paid") which gives sub-totals of "Paid" and a Trailing Grand Summary of the "Total Paid"
I need to include a calculation which gathers the Trailing Grand Summary of the "Total Paid"
I tried this calculation: FloatCurrent = FloatCf - GetSummary(Total Paid;Date), but it doesn't work. Still get the value of last Total Paid sub-summary, not the trailing grand.
From within the same table occurrence context, referring to a summary field in a calculation should return the same total as you see in the trailing grand summary. (Getsummary is great for getting subtotals as produced in sub summary parts, but that's not what we seem to have here.)
Since that isn't working for you, I suggest describing what you want to do in more detail. Perhaps your calculation must be defined in a different table and in which case it will ignore your current found set and produce an aggregate value for all related records instead.
"I tried this calculation: FloatCurrent = FloatCf - GetSummary(Total Paid;Date), but it doesn't work. Still get the value of last Total Paid sub-summary, not the trailing grand."
To extract the sub-summary value you use GetSummary with the summary field and the sorted break value.
To get the Grand Total value, "Use the GetSummary function, using the summary field you want the total of as both the summary field and the break field"
Eg: GetSummary(Total Paid;Total Paid )
Ok, but why use getsummary when you can just refer directly to the summary field to get the grand total?
Now you're making me wonder what the point of GetSummary (SummaryField ; SummaryField ) is at all.
I've never known that use of GetSummary was even possible, now that you've enlightened me, I wonder the same thing....
Love you guys!!
But as I woke up this morning I got what Sorsbuster meant: GetSummary (SummaryField ; SummaryField ) or in my case GetSummary(Total Paid;Total Paid ).
It works, as it should.
What I don't get is why you have to create the Summary filed for a sub-summary, but the trailing grand is automatically created and only referencable via GetSummary. Seems inconsistent to me.
But thanks heaps: I got what I wanted!
As we just discussed, you should not actually need getsummary for this--though it will work.
You should be able to just use Total Paid in a calculation expression and get the same result as when you put it in a grand summary part.