There are lots of ways to miter the ends of your 1x8s. Pilou has shown one although his would leave you with an open corner.
Here's another way.
Select the edge of the board. This would be the edge at the corner that would drop on your foot when you saw the real board. Get the Move tool and...
...move the edge back along the face. a 1x8 should be 3/4 in. thick so move the edge 3/4 in. and you'll have a 45° miter. The move distance is the thickness of the board. No fuss, no muss.
This method wouldn't work for a profiled molding but for the skirting around your deck, it'll be perfect. Do it on one skirt board component and copy that component around the deck to make the rest. Use Make Unique as needed so you can edit the length by selecting a mitered face and moving it to the desired length.
Inspecting mirrors is a job I could easily see myself doing.
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