Is that 3-3/4" cut size or to finish 3-3/4"?

I think it is cut size

OK. Here's how I do it.

Remember an octagon is just a square with the corners cut off. So you make a square to create a pattern. I've already graphed it so I know it takes a 9" square.

Fold a 9" square paper in half and then in half again so it's quarter size. Make a mark on each edge 1-7/8" from the folded edge (half of 3-3/4). Connect the two marks diagonally 45 degrees across the corner with a ruler and you should have a 3-3/4" line and when you cut it and open it up you have your octagon.

I hope this makes sense.

You are awesome! It worked really well. Thanks so much. Your moniker is so appropriate---you have tools!

You're welcome. Yes, I have more tools than most men but that's another story. Good luck with your project.

Linda, I was with Redmary when she tried your formula and it is perfect. The quilt pattern she want to make using the octogon center is more than I would attempt, but when she is successful, I may have to give it a go...it's really cool. Sorry I don't have a pic or link to share...but will bug her to at least post the first block she makes.
thanks again,
V.