Tds rate (actual or as per 15ca)

Hello Everyone,

We are a Pvt. Ltd. company. We are submitting some foreign payments with 15CA. We consider a rate of RBI for calculation of payment amount and TDS. When actual payment made by bank, rate of conversion is different.

As both rates are different, Which rate should be taken to calculate while payment of TDS to Government ? from 15CA or Actual ?

Now we are using Actual rate of transaction considering 15 CA is approximate calculation.

Example TDS is 100 USD 
15 CA created on 15Oct with rate of 68/- so TDS will be 6800/-

Bank processed the transaction on 20Oct at 69/- so TDS would be 6900/-

What should we consider and why?

Please help me on this topic. 

Thanks in Advance