Calculating an amount to add to an amount calculated in a report

[hilian]

In a report I need to calculate a total amount spent for a time period (a quarter) and the remainder available for the rest of the year. If I’m creating a report for the first quarter, then the problem is easy, simply subtract the amount spent from the amount budgeted for the year. The result is what’s left for the rest of the year. For the second, third and fourth quarters, I need to add the amount spent for past quarters to what was spent for the quarter covered by the report and then subtract that amount from the amount budgeted. How do I do that?