Calculate tax not by line items

 Hi there,

   Is there a way to calculate Tax by sum of total not by each line item in Sales order?

For example: when i create Sales order with Item A   price 5 tax 2.25 (only assume)

                                                                Item B price 6 tax 2.35 (only assume)

                    So in Sales Order Header : so total sum Price 6+5 =11 Tax 2.25+2.35= 4.60 

      But what i really want is to let the program calculate Tax only in Header so sum price 6+5 = 11 then calculate tax = 4.5 (only assume)

      The problem is that there is the diffrence between calculate tax in line item and only in total in header.  So i need to let the program calculate in total sum not in line items.