Tax Calculation at Order Entry

Dear All,

My client want to see the tax getting calculated at the moment he enters the Purchase Order. May I know how to accomplish the task?

I have entered both tax explanation code as "S" and tax rate and area as "DM" where we have set value as "7%" of order total.

But, still I am not able to get the tax amount calculated when I take "Order Summary" from row exit.

Thanks

Vimal