R12 FIFO Material Overhead calculation based only over Material cost

Hi
In FIFO Costing we are using Material Overhead with basis type 'Total Value'. The problem is that the calculation is based on the total value of the good (Material+Resources+Overheads) to calculate the MOH amount.

We would need to implement a basis that the MOH is calculated as a percentage only on the material cost, excluding manufacturing costs.

Is there anywork around possible? Does anyone knows a solution or suffereing similar problem?

Any help is welcome

Thanks in advance

Alex D