Job journal totals

I have this code in the onopenpage of job journal page 201 which works fine to update changes in job journal.
But I also want it updated live, while the user is doing the changes. I've tried placing this code in onaftergetrecord but it just sets it to 0 and makes it worse. How do I make this work?


Totalcost := 0;
REPEAT
Totalcost := JobJournalLine."Total Cost" + Totalcost;
UNTIL JobJournalLine.NEXT = 0;