Tax percentage and rate in PO out put

Hi Gurus

We are developing the smart form for PO.I have given logic for all the feilds in the PO, but im not not getting the tax logic how to link it up.we are not using CIN.we have only taxes like CST, VAT and service taxes for different percentages.So kindly tell me how to fetch the Tax rate and percentage linking from PO.

Thanks in Advance......

Sri.