Currency options : Underlying price to calculate Implied volatility

#1
In USDINR option chain at nseindia.com , what is the underlying price (spot Usd-Inr exchange rate) used to calculate implied volatility.
I have tried RBI reference rate (today/yesterday) and one month future price , but nothing seems to fit in.

Used calculator at option-price website , with interest rate 10% , dividend yield 0.


regards
 

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