Tax Calculation On Net Amount

Dear All,

Please suggest me on the below matter,

I have configured Tax Procedure for Input Tax.When I am passing the invoice through FB60

Tax is calculating on Gross bill amount.

Suppose on rs 1000/= I have to collect 10.3 % tax.

Accounting Entry:

Expenses A/C DR 1000

Service Tax A/C DR 103

Vendor a/c CR 1103

Now when I am passing the entry, tax is calculating correctly if I give Rs 1103/= in expenses A/C.

But I want to give Rs 1000/= in expenses A/C, & tax will be calculate automatically on it.

Please tell me what changes I have to do.

Regards

Amitava