Scenario:
User is trying to export his goods out of India by paying GST.
<< I believe some development is about to release in 10.0.13 to export goods with GST>>

I thought of sharing customer expectation related to this process and follows:

There are 2 type of exchange rates followed:
1. Peg Rate or M Rate defined by user to transfer goods with in his company or customers
2. G Rate - It is defined by India Custom authorities for Import Exchange Rate and Export Exchange rate atleast twice in a month

So, Invoice Voucher should calculate exchange rate as below:
1. Legder account related to payables, receivables, in-transit should calculate based on Peg Rate or M Rate as defined by user
2. GST Payable ledger account should calculate G Rate as defined by India Custom releases.

I believe we can define new exchange rates in Forex module but not sure above behavior will be covered.

It should be standard behavior of India Export Behavior process and not to be achieved through Customization.