How to charge GST in old gold manufacturing

sir,
As a retail jewellers during vat regime whenever customer brought his old gold for manufacturing we always charge vat only on the excess weight against his old gold.
Suppose customer gives order to manufacture Bengals in 150gms against his old gold 100gms.
Here his order form was made in this manner
150.000 gms
old - 100.000 gms
50.000 gms×gold rate=xxxxx
making =xxxxx
Taxable value=xxxxx
Vat 1.20% =xxxxx
Total to pay =xxxxx
Then As we still follow G.11/12 act the order was manufacture thru goldsmith by issue & receipt vouchers and thus delivered thruTax invoice.
so customer has to pay tax only on excess that is 50gms.
please clarify the same has to be maintain or any change in GST
Thanks! Dinesh Banthia