How can I allow Tax Rates to populate in summary area on PO's and SO's?

How can I allow Tax Rates to populate in summary area on PO's and SO's?

Dear Sir/Madam

I believe I tried posting this before and it was being moderated but then it went missing?

I would like to know how I can cause the automatically generated tax calculation per line to then be calculated in the Tax summary under the sub total area on Purchase Order and Sales Order modules without it then adding an additional 20% tax rate to the Grand Total of the cost?

Yours Sincerely
Andrew Devall BSc.(Hons.)