how to apply the withholing tax (TDS) only on the line amount

Hey all ,

i want to create a PO with some service tax on the item and some TDS . the problem i am facing in here is after calculating the tax , The origin amount for the withholding tax is always the gross amount i.e “line amount+the service tax” . i tried to change it from the formula designer but there is only the option for gross amount in it . i also tried to change the adjusted origin amount in the temporary withholding tax but that doesnt help as well .

please help !!!