You're most likely much better off to use a related table with a portal instead of repeating fields. Among many other advantages, you can then use a summary field with the running total option to do this.
If you insist on repeating fields you'll need a separate calculation field for each repetition:
Repeating Field Running total
Rep 1 Repeating field (specify 1 repetition)
Rep 2 RT1: GetRepetition ( Repeatingfield ; 1 ) + GetRepetition ( Repeatingfield ; 2 )
Rep 3 RT2: RT1 + GetRepetition ( Repeatingfield ; 3 )
and so forth
In FM11 (and I presume any other version with the Sum function), this can be done using a 2nd calculation field.
Number - a repeating number field
RTotal - the running total of numbers (a repeating calculation field with the same number of reps as Number)
1 ) Assuming you already have the Number field you want the running total for, you will need to create RTotal and set it to = 0
Step 1) is required otherwise step two will cause a Field Not Found error, as it is recursive.
2 ) edit the calculation for RTotal entering:
If ( Get ( CalculationRepetitionNumber ) = 1 ;
RTotal[ Get ( CalculationRepetitionNumber ) - 1 ] + Number[ Get ( CalculationRepetitionNumber ) ] )
Not sure if this is relevant to 5.5 but will hopefully help anybody else brought here by Google (like me).