WCT and Service Tax on EPC Contract

how to Calculate 8%VAT Set off in work contract?

If in case we purchase input vat @ 4% and input vat @ 12.5% & we charge to our customer 8%output vat .....how to get set off.......we get 100%set off in this case?

Dear Sir

First  you clear me the defination of EPC . and also clear me if any principal hand over any to a  contractor,  the  job which containts design & engineering , supply of all plat , machinery & equipment for site including commissioning spares for the work including performence test of all materials. how  the contractor maintain the book for calculation of wct and service tax . 

Kindly give me some expert advice in the view of WCT and Service Tax

Regards

Sibapada Mukherjee

e-mail- sibo_999 @ yahoo.com.in