How to calculate cumulative figure in report and balance using total cumulative figure

I have one table ClaimDetails and it has entities named RepairMount and MaintenanceCost. I have calculate cumulative amount for Repair Amount using running sum.
e.g Repair Amount      Cumulative
       150                           150
       100                           250
       50                             300

how am i going to calculate balance of the Maintenance Cost using cumulative amount? My calculation formula should be :
Balance = [MaintenanceCost] - [Cumulative]
The figure I had is wrong.
eg. Maintenance Cost = 1000 when i run the calculation, it shows balance =  850 instead of 700. I believe it takes the first number (150) and not the last number (300).
how to make system recognize the last number which is the total of cumulative amount?
please help.....
cujee75Asked:
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jerryb30Commented:
Can you show the corresponding MaintenanceCost data?
How is Balance = MainteanceCost-Cumulative different from
Balance = MaintenanceCost - sum(RepairAmount)?
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FlysterCommented:
You can try:

Balance = [MaintenanceCost] - Max[Cumulative]

Flyster
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