Using a double masthead halyard block introduces friction, because the final fall of the halyard to the deck is often out of line with the others that go to the yard. I suggest that you try two singles, for 3:1 purchase, or a single and a single and becket, for 4:1. If you need more purchase than that, it's more efficient, and better ergonomics, to add a small self tailing winch than to add more parts to the halyard.