Computation of Management Fees
The computation of management fees depends on the contract you have signed. However, if you base the management fees on the standard cost, you will lose out in terms of additional billing.
Considerations for Working Capital
Also, remember that you are using working capital to sustain the payments, and overtime means you need to use additional working capital. This carries a cost in terms of interest or lost opportunity cost, which you should be able to recover.
Impact of TDS on Contractor Bills
Another point to remember is that some companies deduct TDS of 2% on contractor bills, so you will lose 2% of OT as well. If your management fees exclude OT, you may face a cash flow problem because the refund of TDS will come to you 18 months or so down the line.
Effect of Overtime on Worker Deployment
Furthermore, the fact that the workers are doing overtime also means that you are being asked to deploy a lesser number of workers. If you were deploying additional workers instead of paying overtime, your billing would be higher anyway. This is another justification for following the COST + OT method.