A few years ago, I helped a friend to frame a house that had an octagonal window. When it was time to frame the octagonal rough opening, he wanted to play it safe by first framing a square rough opening and later filling in the angled pieces after the window had arrived. His last effort at octagonal framing had to be ripped out because the angled sides were a little off. When I asked if he had used the octagonal scale on his rafter square, he didn't know what I was talking about.
Like my friend, a lot of professional carpenters have no idea what the octagonal scale is or how it works. To them, it's just more meaningless chicken scratch etched into the face of their framing squares. In addition, using the octagonal scale commonly is misrepresented as an archaic technique with no useful purpose, save for laying out octagonal posts.
Octagonal openings can be laid out other ways, but this one is fast and accurate. It also works on any octagon (not just posts) from 1 in. to 67 in. wide. And once the window's in place, it works for drywall, too.
Here's how to proceed: