Trim until it goes on hand tight to about 10:30 to 11 O'clock at most. That's my method with 15 builds so far. It will go the rest of the way to top dead center once you put the wrench on it. Go slow as 7.62x39 said. Metal removed can't be put back and you don't need to remove much if you're already at 9 O'clock.
I sandwich a piece of emory cloth between the barrel and the old receiver stub to remove metal from the barrel shoulder. The stub keeps the sand paper flat for even removal all around. Simply work the paper back and forth.
So who has DCI FAL receivers for sale now?