When cutting your decking boards to fit around your deck posts, the first thing you will notice is that your deck posts are almost never square to your decking boards.
How do you overcome this? You need to use a sliding bevel rather than using a square. A sliding bevel is tool similar to a square, except the blade/ruler on the bevel is adjustable to whatever angle you require. The point of having an adjustable blade/ruler is so that you can precisely mark the angle on which your decking board needs to be cut, to fit neatly around the posts.
As you can see in the photo, the decking boards are not square to the posts and by using the sliding bevel you are able to mark the decking boards to exactly fit to the post. To use the sliding bevel correctly, ensure that the tightening nut is tightened and the handle of the bevel is firm against the decking board. Now push the blade into the post and ensure that the handle remains against the decking board and the blade is touching the whole edge of the post. Now tighten the nut firmly and mark your decking board. That is it – one of the tricks that builders use to ensure joins look amazing.