No, this is not a way that you can accurately calculate this ratio, because trading is often an off-blockchain event. The sequence of events looks something like this:
- You send bitcoins to the exchange, using the blockchain. (Or you send fiat money to the exchange.)
- You make any number of trades internally to the site. The blockchain is not involved, only ledgers controlled by the site are updated.
- You send bitcoins from the exchange to a wallet, using the blockchain. (Or withdraw fiat money.)
Trading volume measurements are measuring step 2 there. That volume is not directly related to the blockchain volume. But, if you have other data too, to help you make sense of it, you could estimate the ratio in question.