Pricing procedure

Dear all,

I want to calculate tax on Total price in Purchase Order currently  tax is calculated on Base price ....

for example :

base price 1500

freight : 100

now when i am add tax code than VAT and Service tax will calculate on Base price like

VAT 5% = 75

if add service tax then 185.4

but i want to calculate like

VAT 5% = 80

if add service tax then 197.76

Thanks,

Mahipal