Mike,
I know you are going to hate hearing this, but I do it all freehand.
Not only that there is a distinct taper from front to back.
My handles are flat scales when I glue them on, the only part that is finished before gluing are the front edges of the scales.
After the glue is set I grind the scales to match the full steel tang, put in my taper, and then hit the corners to make the octagon.