Gstr 3b calculation

As per section 15 (3) (a)  CGST act  2017  , calculation  GST (Tax )   should be  after  deducting  discount  amount  . 

Under section 15(2)&(3) of CGST Act 2017, Subsidies directly linked to the price of goods.

You have to calculate tax after deducting discount ie.Value of Goods - Discount = Taxable value ( for tax calculation)

As per the Valuation Provision of Section 15(1) of the CGST Act 2017, GST is levied on the TRANSACTION VALUE.

Now the computation methodology of TV is given under Section 15(2) and 15(3) of the CGST Act 2017.

Transaction Value excludes any Discounts given before or at the time of the Supply and such discount shall be duly recorded in the invoice.

So your Computation is wrong. You have to compute in the above mentioned way.

As discount is given at time of supply or
agreement of discount providing at time of supply
then it should be deducted from value from supply as per valuation rule under GST
Soo taxable value = transaction value - Discount