I have long been frustrated trying to get a decent fit while installing decking around deck posts. The drawing shows how I finally solved the problem. Before installing the posts, I simply cut a 1/4-in.-deep dado around the post. The width of the dado is equal to the thickness of the decking, and the bottom of the dado is flush with the tops of the deck joists.
I cut the dado by making multiple kerfs with my circular saw. Then I clean up the stray bits with a chisel.
Installing the decking is now a matter of loosely fitting the deck boards to the bottom of the dado. This method saves time and frustration, and best of all, the posts look as though they grew there.
Mike Nolin, Brewster, MA