Finished the picture-frame border on my deck this weekend and I keep going out just to look at it

Did the same thing on my back deck last fall and the marking-off-the-actual-corner trick is exactly what saved me too. I burned through two boards trying to get a 45 to work on a corner that was nowhere near square, then my neighbor walked over, clamped the board up loose, and said 'the deck will tell you the angle, stop guessing.' Scribed the line right off the overlapping board and it closed up like it was meant to be. I also learned to cut the two boards on each corner at slightly diffrent lengths on purpose, so the joint sits flat instead of one edge riding proud. Took me three tries on my first corner to figure that out, but once I did the rest went fast.

One thing I'd add from my experiance: leave yourself more overhang than feels comfortable, like a good inch or so past the corner, then sneak up on the final cut. A couple of my boards ended up needing a second trim pass and if I'd cut closer the first time I'd have been buying replacements. Pressure treated lumber from the big box store is never quite the length they say anyway.

Funny how a project that sits on the list for two years takes two days once you finally start. Mine's still holding tight through a full winter of snow and freeze cycles, no gaps opening up at the corners yet.

I skipped the mitres on mine and I don't regret it. My corners were so far out of square that any 45 was a lie, so I ran the border boards long and butted them into short return pieces at each corner, like a little L. The returns are cut square, so all the ugly angle guessing hides inside the overlap and you never see it. Cost me maybe twenty extra board feet of decking and zero stress waiting to see if a joint would close. One thing I'd add to the one-at-a-time trimming method: I made a story stick instead. Cut a scrap to the exact inside width and used it as a spacer between the field and the border as I worked my way around. Kept the reveal consistent even on the sides where the framing had wandered. Faster than measuring each board, and I stopped second-guessing my tape.

Also, my border sat a bit proud of the field by the end, since the original boards had weathered down over the years. Ran a hand plane across the high spots once at the very end rather than fussing over each joint individually. Took ten minutes. Agree with the neighbor in the earlier reply that the deck tells you the angle. I just decided to stop asking it questions altogether.

The meticulous preparation of the corners is certainly important, but I have found that the fastener selection often determines the ultimate success of the entire visual assembly. When securing these delicate mitred joints to the joists, using standard structural screws is fundamentally flawed beacuse they always penetrate too far into the decking. I opted exclusively for trim screws of a very shallow length to ensure that the fastener head sits completly flush against the surface plane without compromising the aesthetic smoothness of the wood.

Did a border like this on my sister's deck two springs back and the thing that finally made my corners look decent was swapping the dull blade on my miter saw for a fine-finish one. The old blade was tearing out chunks on every cut and I kept blaming my measuring. Fresh blade, clean edges, and suddenly the joints closed up. Also used an offcut of the actual decking as my spacer instead of a plastic one, since the boards had a bit of cup to them and the scrap sat truer. Curious what yours are framed with, mine was pressure treated and I had to pick through the pile at the yard to find boards that weren't twisted.

The trim screw thing is real. I used regular deck screws on my first mitred border and the thin end of every corner board pulled down and split within a month, so I redid the whole thing with trim screws set by hand near the points. Something else that helped me: I put a bead of exterior glue on the joist under each mitre before screwing. Two winters later the corners haven't gapped at all, even though the field boards have shrunk a bit around them.