script new record creation and also sum data

I have made a test file for myself to work out some scripting which I want to include in a larger budget accounting file I want to make.

The file has one table 'main', with the fields

budget category [indexed]

transaction

transaction date

transaction month [calc, indexed]

transaction year [calc, indexed]

and an occurrence of the table 'main' called 'main for scripted total'

related to the 'main' table by budget category, transaction month and transaction year

What I would like to do, ultimately, if I can figure out how to do it, is script the creation of new records and individual budget category totals and subtotals. I want to set these fields so the data is 'permanent' and so can be used elsewhere and also gives you a progressive set of totals to see as records are created.

the present script is create new record and set monthly budget category total

Goto layout ["main' main]

New record/request/

Pause/resume script

#fill out data here

Set field[main::transaction month total; Sum(main for scripted total::transaction)]

What happens is, for the first record in a month/year for a budget category, the transaction month total is empty, and the 'transaction' data entered for this record is not summed with future records. When a second, and subsequent records for a particular budget category and particular month/year, the script works as desired, the field is set and the total increases record by record.

My question is how do I properly setup and script the data for the 'first' record of a month/year, and in this case, have its data sum with subsequent records?

Help much appreciated