calculating sum for a new data created

[qster]

hi,
just wanted to ask if anyone could help me on this problem.
I'm trying to create a "sum" formula from the formula fields to calculate total time (in numbers strings)

I had used:
WhilePrintingRecords;
//get the login date of the next record
DateTimeVar nextLogin := Next({Consolidated.START_DATE});
//get the Date diff between the logout date of this record and nextLogin
NumberVar myDateDiff := DateDiff("n",{Consolidated.END_DATE}, nextLogin)/60;

this gives my total time between end and next start ticket in hours (interger) format.
What I attempting to do was to get a running total for the new formula field which I named {@hook time}.

Now when i create a new formula to run a sum of the {@hook time} i get a "the summary / total running field cannot be created"

Does anyone know of a solution to this problem?