Gst on Composite supply

How does the IGST on Composite supply take place . 

Suppose the Value of goods is Rs 1000

Discount is Rs 100 and Freight is is Rs 50. IGST rate is 18 %  then as per my understanding IGST figure will be as follows: 18% (1000-100+50)= Rs 171.

Now my question is after the introduction of e_invoice will the same formula be applicable ?