I think you should set HBD equal to 1 and calculate the implied volatility of Hive only.
HBD is set to 1 for the witness price feeds. Hive price is derived as an exchange rate with regards to HBD = 1.

Check me out witnessing and setting prices as an example:

https://www.hiveblockexplorer.com/tx/e5d5a575446c8b0fd1750eff8f4855a71ad95a77

It looks prettier if you look at my last feed publish here:

@blue-witness

:D