Exchange Rate for Export Proceeds

follow Customs Exchange Rates notification;

https://www.cbic.gov.in/Exchange-Rate-Notifications

use latest one applicable to your transaction period.

ex:Exchange Rates Notification No.40/2019-Custom(NT) dated 06.06.2019.  [Effective from 07th June, 2019]

You may book export entry considering Bank Exchange rate as on the date of Bill of Lading.When the actual remittance comes from the buyer or buyer bank you may consider hoghr or lower then the exchange rate at which entry was made as gain or loss.