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.....
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Can you show the corresponding MaintenanceCost data?
How is Balance = MainteanceCost-Cumulative different from
Balance = MaintenanceCost - sum(RepairAmount)?

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You can try:

Balance = [MaintenanceCost] - Max[Cumulative]

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