You have the right idea, but you need to be careful with how you initialize data and increase it's value. In the first example, you are adding the opening balance with the detail records' balance. The effect of this will be that the opening balance gets added over and over and over. Your final result will be huge (and very wrong). You need two formulas. The first initializes the varialbe to the opening balance and this would go in the group header (or report header). Then take the second part and make it its own formula as welll (Running = Running + Invoice amount) and put this in the details section. That way, the invoice amount is added to the previous balance for every detail record. And you also don't have the openning balance being added in multiple times.